Belgian police searching for a suspect in the Brussels airport attack have seized a nail-packed bomb, chemicals and an ISIS flag in raids.
ISIS claimed responsibility for coordinated bombings that killed at least 31 people at the airport and the subway on Tuesday morning. U.S. counterterrorism officials said the claim appeared genuine.
The terror group's statement, posted by the ISIS-affiliated Amaq news agency, said Islamic State fighters carried out the bombings with "explosive belts and devices." It also claimed that the suspects "opened fire" before detonating their bombs at the airport.
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Belgian police confirmed that three men in a photo taken from surveillance video — pushing luggage carts with bags through the airport — are considered the suspects.
Two of the men, who were dressed in black, are believed to have died when the bombs exploded. A third, wearing a black hat and a white jacket, is being sought. Police released his photo on a wanted poster with the question: "Do you recognize this man?"
The names of the men were not released.
In the hours after the attacks, investigators carried out raids in Schaerbeek, where they found an improvised explosive device containing nails, chemicals and an ISIS flag.
It was not clear how the raid sites were connected to the suspects.
U.S. intelligence officials said their working theory is that Tuesday's attacks were carried out by ISIS militants and sympathizers who were part of the same network that conducted the Paris attacks in November.
The Paris operation was led at one point by Abdelhamid Abaaoud, a Belgian-born jihadist who fought in Syria for ISIS and was killed in a raid last fall in Paris.
The same group also was involved in the attempted attack on a train from Amsterdam to Paris last August that was foiled by U.S. service members, and a failed attack last April on a concert hall in a Paris suburb.
One U.S. official said analysts believe the plot was already in motion, but may have been sped up after authorities arrested suspected Paris attacker Salah Abdeslam, who has information about the network's activities in Europe. Belgian authorities have said he is cooperating with interrogators, and U.S. officials say they have no reason to doubt that.
The group includes operatives who got military training with ISIS in Syria. Officials believe the group is using encryption to mask its conversations and movements. It also is taking advantage of a fragmented Belgian counterterrorism regime that is hamstrung by restrictions, such as a ban on night raids of private homes.
They have managed to employ TATP bombs without blowing themselves up, officials say, which is a testament to their experience with suicide and vehicle bombs in Syria and Iraq.
Annick M'Kele, Alexander Smith and Cassandra Vinograd contributed. This article originally appeared on NBCNews.com.