Fingerprints: Discovery to Identification

Take a walk through a fingerprint case study -- from discovery to identification -- get an idea of what the fingerprint process looks like, from the perspective of a real crime scene investigator.


Now, this case is an example, but know each case is unique and processes used in this case, may differ from those used in other cases.  However, all of the content is real, with only a redaction of some details as is required for such a case study.  

The Call - Bank Robbery

At 8:00p.m. I was working in the office, catching up on reports, with the radio on scan, I heard the call comes in for a Bank Robbery in progress -- single male with a firearm -- come across the air.


District patrol units were responding, Robbery had been paged, but knowing a forensic request would be coming, I set aside my  work for later, jumped on the call via radio, and begin making my way to the scene.  That being said, there was no rush, no need for lights or sirens, and at times that is a challenge to resist. In forensics, it is rare that rushing ever helps, and almost always causes problems. 

On arrival, I met investigators, and began to take a look at the scene


The Scene

When I arrived, I noticed the scene was contained, scene tape and a locked door prevented anyone from entering.  As the officers within open the door, they took my badge number, and I looked at the scene.  Inside a detective was talking to a witness, while another witness is behind the counter was writing their statement.  The officer told me the desk area with the red chair is where it occurred, and they put a plastic bin over a $20 bill that was dropped by the suspect as he fled the scene. When I looked at the bin, the investigator continues to say they didn't want anyone to step on the bill, or touch it before forensics arrived.


The officer then continued to provide a summary of the event: One suspect entered the bank, went straight to the counter with the chair.  The suspect presented a note to the clerk, showed a gun in his jacket, and fled after unknown amount of cash was given. Overall, it is estimated the suspect was in the bank for only a few minutes.  Officers arrived three to five minutes after the suspect departed, but the scene had since been locked down, all officers wore gloves, and the area involved has not been touched, outside of door being opened and the plastic bin being placed over the bill on the floor.  


From there I approached the Detective, who finished with the witness and broke away to speak with me.  They advise a similar occurrence as the officer mentioned, and related they just had stills from the video surveillance emailed to them.  It appeared the suspect was wearing a thick winter jacket, balaclava, and winter gloves. As such, the bill on the floor was no longer of interest, and there is no interest in fingerprinting the door or other surfaces.  The detective thanked me for coming out so quickly, but related there may not be much for me to do.  


The Exhibit

After speaking with the officer and detective at scene, and both mentioning they don't think there was much for me to do. I realized they were both overlooking, or perhaps not perceiving the value of, a specific item.  

Even though the suspect was wearing gloves, only in the bank for minutes, there is a significant value for the robbery note left behind.  The detective had planned to collect and submit the note, through internal channels, but did not realized the value. 

Many people don't realize how spectacular paper is for fingerprints. They often think it is a poor surface, but it is one of the best at times. Given, that the robbery note was brought in by the suspect, there's was a good chance the paper was handled before the gloves were put on! 


The Process

Having attended the scene, I was able to ensure from start to finish the process was done well for this exhibit.  It was dropped on the counter, as the suspect took handfuls of cash, and had not been touched by anyone since it was dropped.  

After making notes and photographing the exhibit, it was collected with a freshly gloved hand, and placed into a paper bag for short term storage.  On the exterior of the bag, I quickly wrote the exhibit number, case#, date, time, quick description of the exhibit, and my regimental as the officer who collected it. 

Only an hour had passed since the initial call, but the scene examination was competed. The investigators remained to continue their investigation: speaking with witnesses, collecting video surveillance, and making inquiries in the area. But I had an exhibit that required processing, as it may hold a key piece of evidence. 

Back at the office, I considered what may be best for this exhibit.  The exhibit is more than just a note. It is a potential source for: fingerprints, DNA, hand writing analysis, statement analysis, chemical analysis of the ink, or physical match of the torn edges to the original source of paper it is from.  But right now, which is the most significant?

Depending on the circumstances of the case, this answer may vary.  But in this occurrence, I had an unknown and armed person, who has used threats of violence to commit a crime.  This person posed a threat to the community, and needed to be identified and apprehended. As such, establishing identity of the suspect was the immediate goal. Given what I had that narrowed the focus to fingerprints or DNA. Knowing that I may process for DNA, after processing for fingerprints, but not the other way around, fingerprinting was the best choice. 

However, the next choice I made, may have been the most important choice of all -- how do I fingerprint the paper?


The Fingerprint

Finding fingerprints on any surface requires a consideration for two basic things: the substrate and the matrix. 


The substrate is the surface you are looking for fingerprints on, and the matrix is the composition of the fingerprint itself. For each there are multiple considerations, but for fingerprints on paper, the choice is quite simple.  


The substrate is paper, and the most significant consideration is  the fact it is porous, meaning liquid can be absorbed into, or pass through it.  Therefore, the matrix of the fingerprint may be on the surface, or may be absorbed into the thin sheet paper, so treating the surface only may not reveal fingerprints. 

The matrix is the next consideration, because if we wish to find the fingerprints, we need to know specifically what to look for.  A traditional matrix, what fingerprints are commonly composed of, is a mix of sweat, oils, lipids, amino acids, and other contaminates. As any fingerprints are completely latent, if present, at this time.  We can only assume they are composed of the traditional matrix, if present, as we have no evidence of any other matrix being present. 

Given those considerations, the best choice of development medium is an amino acid reagent, which has been proven to work well with paper exhibit specifically.  Now, for amino acid reagents, there are three basic choices: ninhydrin, DFO, and indanedione zinc chloride. In this case I chose indanedione zinc chloride, and the exhibit was saturated and placed into a chamber with high heat (~80 degrees Celsius) and high humidity (~60%) for ten minutes.  

The indanedione interacts with the amino acids and creates a pale pink colour, but it is often hard to see the finer details of fingerprints with only indanedione present. 


The Visualization

Although the presence of indanedione alone does not make fingerprints completely visible, it is far more effective than both ninhydrin and DFO in detecting the presence of an amino acid matrix, in my experience. However, when combined with zinc chloride, which can be be visualized through luminescence when a forensic light source is used.  This development medium is the most effective process in locating and allowing visualization of fingerprints left on a piece of paper.  

In the images below 505nm (blue/green) light was used and the results were visualized through an orange filter.


The Enhancement

Once I had done as much as I could to find, visualize, and document the fingerprints.  The next step was to see if I could enhance the photographs and allow for the maximum amount of detail to be seen. This could done many ways, but the most common tool for me is Adobe Photoshop, which allows for the enhancement of the image.

Not all fingerprints are sufficient for individualization, as seen above.  There were four impressions found on the paper, but impression R-1 was not sufficient, and impression R-4 was the most detailed impression available. However, this could not be seen by myself until all were enhanced. 


The Search

Once the fingerprint has been found, the next step is the search.  If there is no potential suspect to compare the prints to, they are submitted to AFIS (Automated Fingerprint Identification System) for search. Now the name seems to suggest that the identification is made by a computer, but there is not true, and likely will never be true for many reasons.  


The AFIS system is a biometric system, which uses digital imaging to store, analyze and compare fingerprints. The system does not make identifications, but rather it functions as a database and narrows searches to a list of possible candidates, when an impression is entered and searched using a unique algorithm.  


The Results

For the search of impression R-4, which was the only one submitted as part of a rush process that existed for serious cases. There was a "hit" to a suspect yielded.  Again, this does not mean the computer made the identification.  What had occurred is a technician, who is also a latent print examiner, conducted the search and from the list of possible candidates individualized the impression to one person.  This individualization was then verified by a second latent print examiner and reviewed by a supervisor. At that time, I was notified by AFIS of a "hit" for impression R-4.  

At that time, I attended AFIS and picked up a Hit File, which contained the impression I had submitted, as well as a sealed envelope, and fingerprint cards for three separate individuals.  This is done to ensure no bias is given to myself, as I had to conduct my own analysis and comparison of the impression, and reach my own conclusion regarding identification.  The fingerprints I was provided were from the suspect identified by AFIS technicians, and two other randomly selected individuals. 

The Identification

Once I had the hit file, I returned to the office and began my work.  With each impression, beginning with R-4, before any comparison could take place, I needed to conduct my own analysis.  


The identification of fingerprints follows an internationally accepted practice referred to as ACE-V. In the ACE-V process, the analysis is the first stage, followed by comparison, evaluation, and verification.  But the analysis involves ten separate aspects of analysis: anatomical factors, substrate, matrix, development medium, deposition pressure, pressure distortion, level 1 detail, level 2 detail, level 3 detail, and tolerance. 

After completing my analysis of the impressions, I moved to conducting my comparisons and evaluation stages where I formed the opinion that R-2, R-3, and R-4, could all be individualized to the impressions from a single individual.  

Once I was completed, I packaged all my notes and conclusions into a sealed envelope and provided the hit file to an immediate supervisor, who then assigned the file for verification to another member of the forensic unit who was also a qualified latent print examiner. At that time, the second member conducted their own independent analysis, comparison, and evaluation for the impressions in the file. Once completed, they sealed their notes and returned the file to the supervisor. 


That is when the supervisor is the only person who is permitted to open the sealed envelope from AFIS and the sealed notes from both examiners, to review and determine if all reached the same conclusion.  Only when all can be verified to have made the same conclusion, can the individualizations be referred to as an identification. This process is required to be done in this way to act as a quality assurance measure and ensure false identifications are not made!  

In this case, all analysis and conclusions agreed, which resulted in the identification of a suspect.

R-4 Friction Ridge Chart.jpg

The Notification

Once a suspect was identified, the investigation does not end there.  All we had was a connection between a person and the piece of paper the note was written on.  

Does it mean that suspect wrote the note? - No

Does it mean that suspect robbed the bank? - No


All the fingerprint provided was circumstantial evidence that the suspect had, at some point in time, touched that piece of paper. However, in this case, which is very rare, the timeframe from me attending the scene to a suspect being identified by myself was 5 hours and 34 minutes.  Normally the process takes days or weeks, and you can see why, given the processes involved.  

However, this was a good piece of evidence, and when the suspect was located, through other investigative avenues and evidence, it was believed they were the culprit. As such they were arrested and charged accordingly. 

So What Happened in the End?

For me, I received a subpoena to attend court and a request for an expert report. After writing a 16 page friction ridge analysis report, with 12 additional pages of appendices, which was disclosed. I sat outside the court waiting to testify to my evidence. However, at that time, the Crown Prosecutor came out of the courtroom and said the words I always loved to hear: 

"You're no longer needed, the defence is now entering a guilty plea"