A day after terrorist attacks in Brussels claimed by ISIS killed at least 31 people and wounded at least 270 others, police continue to search for a suspected accomplice.

The man in question, wearing a hat and light-colored jacket, was seen with two suspected suicide bombers on closed-circuit TV at the Brussels airport Tuesday morning, shortly before two explosions went off at the airport and one bomb was set off at a metro station.

Two Suspected Attackers Identified, Another Pursued By Police

Belgian authorities say they have used fingerprints to identify two of the deceased suspects as Belgian brothers Khalid and Ibrahim el Bakraoui.

Belgian federal prosecutor Frederic Van Leeuw said that Ibrahim, 29, was at the airport, while Khalid, 27, bombed the metro station. The two Brussels-born brothers had a criminal record but no previously known connection to terrorism, Van Leeuw said.

Ibrahim was sentenced to nine years in prison in 2010, after shooting at a policeman during an armed robbery, while Khalid was arrested in 2011 for gun possession and carjacking and sentenced to five years in prison, NPR's Dina Temple-Raston reports, citing several U.S. and European counterterrorism officials. It's not clear how long either may have actually served in prison.

A taxi driver who transported Ibrahim el Bakraoui and two other attackers to the airport helped police investigate the attacks. He came forward after a photo of the men was released to the public, Dina says.

Van Leeuw said the taxi driver gave police the address where he picked up the three men. That prompted a series of raids on houses in the area.

The taxi driver also told police that the three men had a bag they left behind when they traveled to the airport, because it wouldn't fit in the cab, Dina's counterterrorism sources say. That bag was suspected to be a nail bomb discovered at the apartment.

In their raid, the police uncovered bomb-making materials, including several kilograms of explosives, 150 liters of acetone, detonators and a suitcase filled with nails, Van Leeuw said Wednesday morning. They also found a computer that contained a document apparently written by Ibrahim el Bakraoui.

Police arrested and interviewed several people in related raids, but are still looking for a key suspect.

The subject of the manhunt can be seen on surveillance camera footage from the airport. The most widely shared image shows Ibrahim el Bakraoui in the center, the Belgian prosecutor says, and an unidentified airport attacker on the left, who officials say is now dead. The man on the right — in the hat — is the one being pursued by police.

That man, who has not been named by police, took into the airport a bag with a large explosive weapon — larger than the two bombs that did detonate at the airport, Van Leeuw said. But the weapon malfunctioned and did not go off until the bomb squad arrived after the attacks were over.

Abdeslam's Arrest May Have Affected Timing Of Attack

Some news outlets are referring to a potential link between the attacks in Brussels and the Paris terrorist attacks of Nov. 13, which killed 130 people. These reports, however, have not been confirmed by prosecutors or any other authority.

The attack comes in the wake of a high-profile arrest related to the Paris attacks. Salah Abdeslam, a suspect in the attacks who had been on the run for months, was captured in Brussels last week. He is set to appear before a pretrial court Wednesday in Brussels.

Authorities are still looking for Abdeslam's accomplices in the November attacks — including one man, Najim Laachraoui, whose identity was only recently revealed.

Laachraoui is suspected to be the maker of the bombs used Tuesday, Dina reports. Her counterterrorism sources say Laachraoui's fingerprints were found on the luggage bombs used in the Brussels attack as well as explosive vests that were used in Paris.

Abdeslam himself may have been planning on participating in the attacks on Brussels, Dina says.

She reports that counterterrorism officials "say they now believe that Abdeslam wanted his lawyer to announce he was cooperating publicly so others he was working with could move up the timetable of their attacks." She adds:

"Officials are investigating, but believe that the attacks were supposed to happen closer to Easter and were supposed to be much bigger. There are still a number of cells they believe are preparing strikes. They believe the bomb factory they raided last night is just one of several sprinkled around Belgium."

Belgium And Allies Mourn, Respond

Investigators and police in France and Belgium have been cooperating in the fight against terrorism. Following the Brussels attacks, the French interior minister announced 1,600 additional police and soldiers would be stationed at train stations and border crossings.

A number of countries have increased security at borders and transportation sites after bombs went off at Brussels' Zaventem airport and Maelbeek metro station Tuesday morning.

The U.S. State Department issued this travel warning Tuesday:

"The State Department alerts U.S. citizens to potential risks of travel to and throughout Europe following several terrorist attacks, including the March 22 attacks in Brussels claimed by [ISIS]. Terrorist groups continue to plan near-term attacks throughout Europe, targeting sporting events, tourist sites, restaurants, and transportation."

Approximately a dozen Americans are known to have been wounded in the attack, a State Department spokesman says; no Americans have been confirmed among the dead.

Speaking at a press conference in Argentina, President Obama expressed "extraordinary sorrow" for the attacks in Brussels. He said the U.S. will offer any assistance that it can in the investigation. "We will also continue to go after ISIL aggressively until it is removed from Syria and removed from Iraq and finally destroyed," he said, using another term for the Islamic State.

"This is my No. 1 priority," he said later. "My top priority is to defeat ISIL and to eliminate the scourge of this barbaric terrorism that's been taking place around the world."

The Brussels bombings follow several other violent attacks that have hit cities in Turkey, Africa and the Middle East in the past 10 days.

Turkey's president, meanwhile, said Wednesday that Turkey deported Ibrahim el Bakraoui from Turkey to Europe last year, and warned Belgium of the man's terrorist ties.

Belgium is observing three days of mourning, and at midday Wednesday (7 a.m. EDT), a moment of silence was held for the victims in Brussels.

The king and queen of Belgium have toured the airport that was targeted in the attack, met first responders who aided victims and visited some of the wounded in a local hospital.

In Brussels, Trying To Resume Daily Life

Belgium has raised its terror alert to 4, the highest level. The Zaventem airport in Brussels will remain closed on Wednesday and Thursday, and possibly longer, as the investigation continues.

But the nation's capital is not on lockdown. The metro is running a limited service, and many people returned to work as usual on Wednesday morning.

On Morning Edition, NPR's Soraya Sarhaddi Nelson said that Brussels residents were defiant Tuesday night, gathering at the Brussels Stock Exchange to mourn the dead and express national unity.

On Wednesday, Soraya says, fewer people than usual were out on the city streets. But those whom she spoke with said they didn't plan on halting their lives out of fear.

"We have a world war right now, and ... life has to continue," one commuter told Soraya. "We cannot change anything, unfortunately."

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