They call him "The Black Cloud."

Acting Sgt. Marc Levesque earned the nickname early in his career as a forensic identification officer after he investigated a different death for 28 consecutive shifts.

Levesque has worked in the Ottawa police forensic identification unit for the past eight years and has attended the scene of more than 100 suicides and numerous fatal collisions, including the OC Transpo bus-train crash that killed six people.

He has also investigated 11 homicides, the most recent being last fall’s shooting of Cpl. Nathan Cirillo at the National War Memorial.

And, he has seen some gruesome scenes and been uncomfortably close to unspeakable violence, sometimes for weeks on end.

"You can’t unsee what you’ve seen," said Levesque.

Developing the narrative of a crime

But in order to do his job, he blocks out the horror by focusing on the science and sticking to a routine.

"I process everything the same way, it enables me not to forget any steps and allow part of my mind to concentrate on the details."

Levesque is one of 24 Ottawa police forensic identification officers. When he shows up at a major crime scene, he immediately puts the area in lockdown.

Yellow tape goes up and all police, except for other forensic officers, are barred from the scene.

It's Levesque’s job to gather and analyze the evidence that "develops the narrative" of how the crime was carried out and ultimately find the evidence that points to the perpetrator.

Impossible not to leave behind DNA evidence

This soda pop can was collected from a break-in scene and police used an ultraviolet light to reveal fingerprints. (CBC) Before he can begin the painstaking search, Levesque snaps on a pair of gloves and steps into a white plastic suit that covers him from head to toe. Crime scene investigators can't risk contaminating the evidence with their own DNA.

Levesque said criminals may be smart enough to wear gloves to hide fingerprints, but it’s impossible not to leave behind DNA evidence.

"A principle of forensic investigations is that you always leave something at the scene and take something away from the scene."

Forensic officers look for a piece of hair or a drop of blood from a scratch, perhaps a trace of saliva belonging to the suspect.

Only a microscopic amount is needed, but it has to be found. In past cases, Levesque has lifted traces of DNA from a cigarette butt floating in a toilet bowl and from skin cells swabbed from the inside of a pair of discarded gloves.

Inside the forensic crime lab

The Ottawa police forensic crime lab is only accessible by crime scene investigators. This is the first time media has been allowed inside the facility.

The lab is detached from the main Elgin Street police headquarters and it has a separate ventilation system to prevent the samples from being contaminated.

Inside the lab, Levesque shows the CBC how DNA samples are prepared for testing.

The swabs are dried for 24 hours in a forensic dryer to prevent bacteria from developing on the specimens and then the swabs are triple sealed in tamper-proof packaging.

Updated fingerprinting technology

And when it comes to fingerprints, Levesque said Ottawa police have the technology and expertise to lift prints from all types of surfaces.

Once a print is lifted, it needs to be analyzed for hours and verified by two police officers before it is considered a match. (CBC) Fingerprints on metal objects such as guns are put inside a special oven called a cyanoacrylate fuming chamber. A plate of crazy glue is heated inside the chamber and after it vaporizes and cools, the crazy glue sticks to the exhibit and reveals a print.

Once a print is acquired, Levesque said forensic officers can take up to six hours to examine each individual ridge of a fingerprint in order to make a match.

Levesque said he and his colleagues take the utmost care in analyzing evidence. They want to catch a perpetrator, not implicate an innocent person.

Levesque can't control other aspects of the investigation, but when it comes to collecting and analyzing evidence it's his daily goal that reasonable doubt won't be found in his forensic work.

"We pay a lot of attention to detail. We are meticulous and in ident we have high standards, it's called the standard of excellence. You can't miss anything. If you miss anything, it's on you."