BRUSSELS, Belgium — Two brothers were identified Wednesday as the suicide bombers behind the Brussels terror attacks as the search for at least one on-the-run suspect intensified.
Authorities said Khalid El Bakraoui, 27, blew himself up in the Belgian capital's subway system while his 30-year-old sibling Ibrahim detonated a bomb at the city's airport. Both had been convicted of violent crimes in the past and had links to one of the Paris attackers.
ISIS claimed responsibility for Tuesday's trio of bombings, which killed 31 people and injured at least 270 others.
Belgium's federal prosecutor said Ibrahim El Bakraoui was the man pictured in the center of a surveillance video photo taken at the airport — but would not confirm the identities of the other two suspects with him.
The man in the light-colored jacket and hat left a suitcase behind and then left the airport, according to officials. He was being hunted on Wednesday.
Authorities also confirmed that the initial airport bomb exploded at 7:58 a.m. local time (2:58 a.m. ET) — with the second following around 10 seconds later.
A massive operation was underway seeking a suspect named by police sources as 24-year-old Najim Laachraoui. Investigators said he had traveled with Paris attacks plotter Salah Abdeslam, who was captured on Friday. It was not clear whether Laachraoui was among the three men pictured in the airport surveillance photo.
The day before the attacks, authorities appealed for the public's help in tracking Laachraoui down. He is known to have used the alias "Soufiane Kayal" and went to Syria in February 2013.
Local reports on Wednesday morning suggested that Laachraoui had been arrested in the Brussels suburb of Anderlect but authorities later denied that was the case.
Both of the suicide bomber brothers had criminal records. In 2010, Ibrahim El Bakraoui was sentenced to 9 years in prison for shooting at police with an assault rifle during a robbery.
Khalid El Bakraoui was arrested for possession of Kalashnikovs in 2011 and had been sentenced to 5 years in prison for "carjackings." He had lived at an apartment in the Forest neighborhood which was the scene of a deadly shootout with police on March 15 during a raid linked to the Paris attacks, according to reports by RTBF and the Belga news agency.
An ISIS flag and extremist manual were seized there at the time, and NBC News later revealed that the name and other personal details of an Algerian who was fatally shot matched those of an ISIS fighter who wanted to be suicide attacker.
Khalid El Bakraoui also allegedly rented a house in the nearby city of Charleroi, Belga reported.
Counter-terrorism officials told NBC News that the airport attack involved three suitcase bombs. Two of them were detonated, while a third did not explode and was blown up by Belgian authorities.
No other weapons or handguns were found at the airport, prosecutors said.
A taxi driver tipped off police that he had picked up three passengers at a home in the Schaerbeek district of the city.
BFM reported that the taxi driver who took the trio to the airport said their luggage was very heavy but that the attackers refused to let him help them with it. NBC News could not immediately confirm that report.
However, two sources told NBC News Wednesday that tests suggested the bombs were estimated to weigh 44 pounds each.
A subsequent search of the residence in Schaerbeek turned up a potential bomb-making factory complete with chemicals used in explosives, nails and an ISIS flag. Investigators said Laachraoui's DNA was found there, along with at an address in Brussels raided by police last week in relation to Abdeslam's capture.
Meanwhile, forensics teams worked through the night at the Maalbeek metro station — which was targeted by Khalid El Bakraoui. Officials said his bomb exploded in the second carriage while the subway train was still in the station.
Questions were swirling over how Belgian authorities failed to prevent the attacks, which came just days after the arrest Abdeslam — Europe's most-wanted man — in Brussels.
Belgium's Interior Minister Jan Jambon said he "can understand that people are scared" knowing there's an alleged terrorist on the loose, but insisted security services were working "day and night" to track him and any other potential accomplices down.
Jambon hit back back at allegations of a potential intelligence failure preceding the attacks.
"I don't think we have missed something" he told NBC News. "It's a difficult, difficult job."
"The people of ISIS are also professionals," Jambon added. "So that means that the job is extremely difficult."
Belgium attempted to ease back to normalcy on Wednesday, with schools open and transport networks running. Still, the specter of further attacks loomed: ISIS warned of more "dark days" to come — "worse and more bitter."
The country was expected to pause for a moment of silence at noon local time (7 a.m. ET). The country's king was reportedly planning to visit the wounded.
"Our darkest day," read the cover of one newspaper. Another simply bore the date — 22/3 — against a black backdrop. Elsewhere in Europe, the carnage was captured in headlines such as "Bloodbath in Europe."
The victims reflected the multinational fabric of Brussels, Europe's capital. Three Mormon missionaries from Utah were badly wounded, along with a U.S. serviceman and his family. Brazilian-Belgian basketball player Seb Bellin suffered injuries.
The attacks continued to reverberate around the world — sparking condemnation from leaders but also stepped up security in cities like New York, London and Paris.
Coming just four months after terrorists killed 130 people in a deadly spree in the Paris, the Brussels attacks saw a particularly robust response from France. The country deployed more than 1,600 additional security forces after the bombs went off in Brussels.
The State Department meanwhile issued a travel alert warning U.S. citizens of "potential risks" to travel in Europe.
"Terrorist groups continue to plan near-term attacks throughout Europe, targeting sporting events, tourist sites, restaurants and transportation," it warned.
This story first appeared on NBCNews.com. This story is part of breaking news, please refresh for updates.