How police use blood to solve crimes

Story by: David Lynch | Illustration by: Jordan Mak


A robber breaks a window to get into a house. They’re working fast, maybe too fast. While climbing through the hole they’ve made, their leg grazes a sharp piece of glass, cutting them. They don’t notice, finish the job, and leave.

It’s situations like these in which blood connects people. The bloodstain pattern analyst (BSPA) arrives, using clues from the blood at the crime scene to get an idea of how it all happened. After they’re done, the forensic cleaner comes, making the scene look brand new. The BSPA uses blood for a wide range of questions regarding the crime, says University of Toronto lecturer Wade Knaap.

“Bloodstain pattern analysis can be used as an investigative tool to assist in determining various things regarding the victim and suspect,” Knaap says. “It can help us figure out the sequence of events, the location of the victim and suspect and the potential weapon that was used.”

Tonya Hackenbrook is a bloodstain pattern analyst, one of three with Peel regional police. She said that to become a BSPA takes years of training. For Peel regional police, the officer becomes a Forensic Identification Officer after training for nine weeks at either the Ontario Police College or Canadian Police College. Then the officer takes three different courses. The officer has to register with a mentor, who will decide when the officer is fit to take the advanced course.

The last step to become a BSPA is to be part of a mock trial, with other blood analysts playing key roles. If the participants in the trial think the officer has shown the required knowledge, they are certified. It took Hackenbrook three and a half years to complete her training.

At a crime scene, the BSPA examine the area for fingerprints and blood.

“In the case of a break and enter for example, the culprit may cut themselves when they enter the home or business through a broken window,” Hackenbrook says. “We would examine the glass for fingerprints and for indications of blood.”

If there is blood at the scene the BSPA collects it using a specially designed swab.

“A swab is similar to a long Q-tip,” Hackenbrook says. “The cotton end is moistened with sterile water and then rubbed on the blood to collect it on the swab.”

This swab gets submitted to the Toronto Centre of Forensic Sciences to hopefully find DNA in the sample. But that doesn’t always happen. Hackenbrook says that when they swab the blood at a crime scene they’re hoping to collect something specific — white blood cells (red blood cells don’t contain DNA because they don’t have a nucleus).

“It is possible to collect a blood sample and not have any DNA in it because no white blood cells are present”

Knaap says the type of testing done at these forensic facilities can be very accurate, depending on how the crime scene is when the BSPA gets there.

“It’s very concise depending on how much information is available, if it hasn’t been disturbed, and if it hasn’t been interrupted,” Knaap says.

After the BSPA are finished, and the police are done at the crime scene, it needs to be decontaminated. David Cadieux, a professional forensic cleaner, says a person with this sort of scene in their house should never try and clean it by themselves. He said that you never know what’s in a person’s blood, and that if someone has a scene like this in their house, they should treat the blood as if they have no knowledge of what’s in it. Despite people generally thinking about homicides as a lead cause of blood splatter, that’s not always the case. Cadieux says that he decontaminates blood for everything ranging from homicides and suicides to household accidents.

“Could be grandma on blood thinners, and cut herself, looking for a band aid, and going through the house and bleeding everywhere. When that happens it’s the same as a homicide.”

No matter what the cause of the scene is, if it’s a significant mess, a professional cleaner should always be called.

Knaap says that blood isn’t the only thing left behind at a scene. The blood could release particles in the air that could also be harmful.

“There’s not only the blood, but there’s potentially other pathogens, biohazard materials, there could be chemicals that need to be remediated as well.”

After all of this, the BSPA and the forensic cleaners will wait for the next call. It could be a homicide, another break and enter or maybe a grandma. Either way, they’ll treat the scene the same.

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