Just days after the terrorist attacks on Brussels, the French newspaper Le Monde reported that one of the bombers’ DNA was found in the Brussels airport where he blew himself up. That DNA was also found on the suicide vests used in the Paris attacks, where 130 people died in November 2015. How is it that investigators recovered DNA evidence from these explosives?

"It’s not easy," says David Foran, the director of the forensic science program at Michigan State University’s School of Criminal Justice. "But it’s not impossible."

"DNA will burn, just like any part of your body can burn."

In Brussels, two blasts at about 8AM on March 22nd ripped through the Belgian city’s main airport. An hour later, an explosion struck the subway station of Maelbeek, in the heart of Brussels. The attacks, claimed by ISIS and believed to have been carried out by five terrorists, killed 31 people and injured more than 300. Explosives were also used in the Paris attacks on November 13th, 2015, when shooters and suicide bombers created mayhem at a concert hall, a stadium, restaurants and bars almost simultaneously, killing 130 people.

After explosions like these, the first step is to collect all the objects from the blast scene that could be associated with the attack, Foran says. In the case of the Brussels attack, these likely included fragments of the explosive devices, screws and nails reportedly used as shrapnel, and the suitcases and luggage carts the suspects are shown pushing through the airport in security footage.

The second step is to bring all recovered objects to the lab and swab them to collect any traces of DNA that have survived the blast. This is where things get tricky: DNA can be damaged by heat.

"DNA will burn, just like any part of your body can burn," Foran says. "It doesn’t like direct flame."

DNA evidence recovered from explosive devices isn’t usually high-quality. It mostly comes from fingerprints left by whoever handled the bombs and the containers, like suitcases and backpacks, where the devices were placed, Foran says. Fingerprints melt quite easily with heat, because they’re made of skin oil. But they often contain skin cells the body sheds throughout the day. Even if the fingerprint melts, skin cells are more durable. Those skin cells are used by investigators to recover DNA evidence — but it’s not exactly high-quality. The cells are dead, so the DNA is likely to be degraded.

To make things worse, Foran says, it’s likely that objects recovered from a messy blast scene have a mixture of multiple people’s DNA — victims as well as perpetrators. The challenge is then to separate the DNAs to identify the suspect or suspects. "It’s like pouring four different colors of paint into a jar and stirring them up," Foran says. "You can’t differentiate them." The odds of success could increase if investigators already have DNA records of potential suspects and can use those to differentiate the multiple DNA traces.

DNA from skin cells is likely to be highly degraded

If investigators are able to zero in on a DNA sample, the last step is to enter it into a database to see whether it matches with existing DNA records. This is somewhat imprecise. Researchers were able to correctly identify who assembled a pipe bomb from its explosive remains only about half the time, according to a 2009 study in the Journal of Forensic Sciences.

Thanks to DNA evidence found at the Brussels airport, Belgian investigators identified Najim Laachraoui, a 24-year-old Belgian citizen born in Morocco, as one of the suspects. It’s not clear how French and Belgian investigators had a record of his DNA in the first place. But it matched with DNA evidence found not only on suicide vests used in Paris, but also in two houses in Belgium. In one of these properties, located in the Schaerbeek section of Brussels, investigators found traces of TATP, a signature explosive used by ISIS in Europe.

Laachraoui is believed to have died in the attack at the Brussels airport. Two other suspects are still on the run.

Correction, 2:46 PM ET: An earlier version of this post stated that David Foran works at the University of Michigan. It is Michigan State University.