BRUSSELS, Belgium — Two brothers were identified by police sources as the suicide bombers behind the Brussels Airport attack early Wednesday as the search for an on-the-run suspect intensified.

Khalid El Bakraoui, 27, and 30-year-old sibling Ibrahim blew themselves up in a departures hall on Tuesday morning. Both had been convicted of violent crimes in the past and had links to one of the Paris attackers.

Image: The three men considered suspects by Belgian authorities

Image: The three men considered suspects by Belgian authorities

ISIS claimed responsibility for the trio of bombings at the airport and in the city’s subway on Tuesday, which killed more than 30 people and injured at least 260 others.

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 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 also allegedly rented a house in the nearby city of Charleroi, Belga reported.

Related: Why Is This Brussels Suburb Home to So Many Extremists?

A massive manhunt was also underway for a suspect named by police sources as 24-year-old Najim Laachraoui, who was pictured dressed in white and wearing a hat in airport surveillance footage next to the two brothers. All three were pushing luggage carts.

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.

Authorities had already been seeking Laachraoui, who investigators said had traveled with Paris attacks plotter Salah Abdeslam. He is known to have used the alias “Soufiane Kayal” and went to Syria in February 2013.

Laachraoui’s DNA was also found at an address in Brussels raided by police last week in relation to Abdeslam’s capture and authorities had previously appealed for the public’s help in tracking him down.

The manhunt and investigation saw raids carried out across Belgium. One in the Brussels neighborhood of Schaerbeek overnight turned up a potential bomb-making factory complete with chemicals used in explosives, nails and an ISIS flag.

Forensics teams worked through the night at the Maalbeek metro 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 IS 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.

Defiance mixed with the sorrow in Brussels as the city struggled to process the deadly assault.

Belgium’s King Philippe told the nation that “the threat we face will be confronted with fortitude, calm and dignity.”

Students gathered late into the night on the steps of the Bourse, where a memorial sprouted up.

The tributes were Belgian in every way — from the flags propped in Trappist beer bottles to the young people quietly eating the city’s famous “frites” (french fries) on the stairs of the Bourse under a banner reading “United Against Hatred.”

Gwendoline Pereira and her boyfriend Kevin Ferelol stood arm in arm at the top of the stairs, silently taking in the scene below.

“It’s a symbol — to be here and show protest,” Ferelol added. “It’s a way to say we’re still here.”