The picture captures three men at the Brussels airport, all of them pushing metal luggage carts. Observers say these men have the trademark demeanor of ISIS bombers.

"Look at their expressions, they're calm. You can say they are almost casual and relaxed in the way they are walking into the airport," says New York Times correspondent Rukmini Callmachi, who wrote about this image. "There is no sign, at least physically, that these people are about to blow themselves up into smithereens."

Callimachi says witnesses who survived the ISIS attacks in Paris in November say they noticed the same calmness in the faces of those bombers, but toxicology reports found that remains of the ISIS attackers in Paris showed no signs of drugs.

"They're calm based on their ideology," says Callimachi.

Both the Paris attackers and the ones in Brussels used bombs made of triacetone triperoxide, or TATP. Callimachi says ISIS has become an expert in the use of this sometimes unpredicatable compound. 

"TATP has very much become ISIS's signature explosive," she says.

Despite the fact that TATP is made from many household products, such as acetone used in nail polish remover and peroxide, which is in hair bleach, it is known the "mother of Satan" because many of the bombers who used the compound in the past have lost limbs.

But Callimachi says ISIS has had unprecendented success with TATP. 

"The problem with it is if you are not skilled in how to make it, the compound is volatile and so it's very easy to hurt yourself," she says. "The fact that the Paris attackers successfully detonated their bombs in seven different locations and now that we see that the Brussels bombers successfully did so in three different instances, twice at the airport and once at the metro is indicative of the growth of the operations wing of ISIS."

But despite the new more predicatable patterns being found among ISIS bombers, Callimachi says it's still very difficult for investigators in Europe to prevent these ISIS attacks.

"They are overwhelmed," she says, "It takes five policeman to be able to provide 24 hours surveillance of one suspect, and we now have hundreds and hundreds of suspects, I don't think they have the capacity."


From PRI's The World ©2015 Public Radio International