ISIS claimed responsibility for the morning bombings at the airport and the subway, which killed at least 31 people. U.S. counter-terrorism officials said the claim appeared genuine — and Belgian officials said an ISIS flag was found along with bomb materials in one raid.
The terror group's statement, posted by the ISIS-affiliated Amaq news agency, said ISIS fighters carried out the bombings with "explosive belts and devices."
Counter-terrorism officials from the United States and other countries told NBC News that the airport attack involved three suitcase bombs. Two of them were detonated by the terrorists, while a third did not explode and was blown up by Belgian authorities. A fourth was reportedly discovered in a nearby house.
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Belgian police confirmed that three men in a photo taken from security video at the airport are the suspects.
Two of the men, who were dressed in black, are believed to have died when the bombs exploded.
The third, wearing a black hat and a white jacket, is the one 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 across the country. Counter-terrorism sources described one site as a "bomb-making factory" and seized the elements of explosive devices.
The Belgian prosecutor's office said a raid in Schaerbeek yielded an improvised explosive device containing nails, chemicals and the ISIS flag.
The attacks came just days after the arrest Paris terror suspect Salah Abdelslam in Brussels -- but Jambon hit back at allegations of an intelligence failure.
"I don't think we have missed something," he said. "It's a difficult, difficult job.
"Our services as professionals," he added. The people of IS are also professionals. So that means that the job is extremely difficult."
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 — although the Belgian prosecutor's office said they had not found any evidence of a link.
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 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 counter-terrorism 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.
Ken Dilanian is a correspondent covering intelligence and national security for the NBC News Investigative Unit.
Richard Esposito is the senior executive producer of the NBC News Investigative Unit, and supervises investigative correspondents, producers, and reporters across all broadcasts and platforms of the NBCUniversal News Group.
Esposito, who began as a copy boy at the New York Daily News in 1977, has more than 25 years of newspaper and television experience. He has overseen investigations and run the metropolitan news operations at two of the largest newspapers in America, the New York Daily News and Newsday, and was most recently senior investigative reporter for ABC News.
He has shared in Peabody and Pulitzer awards, and has twice been a finalist for the Pulitzer. Honors include the 2012 Murrow Award for his work in reporting the death of Osama bin Laden, a 2005 Polk Award for his investigation into the CIA’s network of secret prisons and harsh interrogation techniques, a 2006 Emmy Award and a 1990 Sigma Delta Chi award, among many other awards.
Esposito is the co-author of the books “Bomb Squad: A Year Inside the Nation’s Most Exclusive Police Unit,” and “Dead on Delivery: Inside the Drug Wars.”
Tracy Connor is a senior writer for NBC News. She started this role in December, 2012. Connor is responsible for reporting and writing breaking news, features and enterprise stories for NBCNews.com. Connor joined NBC News from the New York Daily News, where she was a senior writer covering a broad range of news and supervising the health and immigration beats. Prior to that she was an assistant city editor who oversaw breaking news and the courts and entertainment beats.
Earlier, Connor was a staff writer at the New York Post, United Press International and Brooklyn Paper Publications.
Connor has won numerous awards from journalism organizations including the Deadline Club and the New York Press Club.
She lives in Brooklyn, N.Y.
Cassandra Vinograd is a Senior Writer and News Editor. Before joining NBC News, she worked as a London-based correspondent for The Associated Press and specialized in politics, foreign affairs and defense.
Vinograd previously worked as an editor for The Wall Street Journal in Brussels and London.
She has reported extensively from Afghanistan and on West Africa and the Middle East.
Robert Windrem is an investigative reporter/producer with NBC News, specializing in international security.