The explosive found at the house where the suspects in the Brussels attacks stayed is preferred among violent extremists in Europe because it's fairly easy to make and detonate, an expert says.

Explosions on Tuesday at the Brussels airport and in a subway killed 31 people, plus three suicide bombers, and wounded more than 270 people.

Belgium's chief prosecutor has said that investigators found 15 kilograms of triacetone triperoxide, or TATP, along with nails and other bomb-making materials at the house where the suspects in the attacks stayed before going to the airport.

TATP was used by Richard Reid, who tried unsuccessfully to detonate a bomb in his shoe during a trans-Atlantic flight in 2001, and in the Nov. 13 Paris attacks. In France, attackers packed TATP into their suicide vests and wielded assault rifles, killing 130 people.

The Islamic State group has claimed responsibility for both the Paris and Brussels attacks.

Two officials briefed on the investigation say the suspected bombmaker in the Paris attacks was one of suicide bombers in Brussels. The European intelligence official and the French police official spoke on condition of anonymity because they weren't authorized to divulge details of the Belgian investigation.

Jimmie Oxley, an explosives expert and chemistry professor at the University of Rhode Island, said it's not surprising that TATP continues to be used in such attacks, because "we've done nothing to take this particular tool away from them."

TATP is a peroxide explosive that's volatile and sensitive, Oxley said. Even small quantities of the white powder can cause large explosions.

Experts have said that tracing the raw materials can be difficult because they are readily available in pharmacies and hardware stores, though Oxley noted large amounts would likely have to be purchased over time.

It's also relatively easy to create a functional device, and there are many ways to set one off. She said it's intriguing that extremists use suicide bombers, since a timer or trip wire could be used.

The U.S. Department of Homeland Security has called together a group of experts to improve the nation's response to the threat of bombings. Oxley is the technical lead at URI for the center. Another University of Rhode Island professor, Otto Gregory, is working on a sensor that detects TATP to try to stop future attacks.

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Associated Press writers Lori Hinnant in Paris and Paisley Dodds in London contributed to this report.