Here’s what we know about the Brussels terror attacks

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Thirty-one people were killed and 270 injured when terrorists struck Brussels’ airport and the city’s Maelbeek metro station Tuesday morning, in attacks subsequently claimed by ISIS.

Belgian authorities said Wednesday that two brothers known to authorities for their track record of violent crimes, Khalid and Ibrahim El Bakraoui, had carried out two of the suicide attacks: the former in the metro, and the latter at the airport.

One person was arrested Wednesday in relation to the attacks, Belgian state broadcaster RTBF reported, but the identity of the individual was not released.

Belgian counterterror official Paul Van Tigchelt warned Wednesday that there were others involved in the plot who remained at large in Belgium and still posed a threat. Belgian federal prosecutor Frederic Van Leeuw said they included a suspect who had left a bomb at the airport before leaving the scene, which exploded later, causing no injuries.

Here’s what we know so far:

The attacks

Airport: At 7:58 a.m. local time, two suicide bombers struck the departure lounge of Brussels Airport in Zaventem about 37 seconds apart, killing at least 10 people.

One blast took place outside the security checkpoints for ticketed passengers and near the airline check-in counters, according to an airline official briefed on the situation.

Van Leeuw said Wednesday that a third bomb, which he described as the “heaviest,” had been left at the airport by a suspect who left the scene. He said it exploded later, after the bomb squad had arrived, because the explosives were unstable, but no one was injured.

No guns or weapons were found at the airport, Van Leeuw said, contradicting earlier Belgian media reports that a Kalashnikov assault rifle was found.

Metro station: About an hour after the airport explosions, Khalid El Bakraoui detonated a suicide bomb on the Brussels subway at the end of rush hour.

The bomber, who Van Leeuw said was identified by his fingerprints, was in the second car of the train at the Maelbeek metro station.

The station is in the heart of the city, where European Union institutions are based, making it a symbolic target for terrorists. NATO is also headquartered in Brussels.

The suspects

Belgian authorities released an image Tuesday of the three suspects believed to have carried out the airport attack.

The image, taken from surveillance footage, shows three men pushing luggage carts through the airport. In the center is Ibrahim El Bakraoui, and on the picture’s left, a similarly dressed suspect believed to have been the second suicide bomber at the airport.

Belgian authorities said Wednesday that they had not identified the second suicide bomber.

A third, also unidentified man in the picture is the suspect investigators say planted a bomb at the airport that later exploded without hurting anyone.

Authorities have launched a hunt for the man, pictured at the airport wearing a light-colored jacket and black hat.

The man is believed to have been a guide, charged with ensuring the others carried out the attacks, who then left the terminal, according to Van Leeuw and experts.

The Bakraoui brothers were known to Belgian authorities. Khalid had used false identity papers to rent the apartment in Brussels where recently arrested Paris terror suspect Salah Abdeslam was hiding after the November attacks, a senior Belgian counterterror official confirmed to CNN.

Belgian police found the will of Ibrahim El Bakraoui on a computer in a trash can while searching the neighborhood of Schaerbeek, Van Leeuw said.

Raids

A taxi driver who took the three suspects to the airport told investigators where he picked them up in the Brussels district of Schaerbeek, prompting a raid in the area Tuesday that uncovered 15 kilograms of the explosive TATP, screws and an ISIS flag, Van Leeuw said.

Van Leeuw said Wednesday that police had arrested two people Tuesday — one in Schaerbeek, who was still being interrogated, and another who had been released after questioning.

Police raids have continued in the Brussels area, and one on Wednesday in Anderlecht resulted in the arrest of an individual whose identity has not yet been released.

Belgian authorities said that, contrary to Belgian media reports, the individual arrested Wednesday morning was not Najim Laachraoui, recently identified as a suspect in the investigation into the Paris attacks.

Victims

The first victim to be identified is Adelma Marina Tapia Ruiz, who was killed in the attack at the airport, according to Peruvian state news agency Andina.

Originally from Peru, Tapia Ruiz had lived in Belgium for six years and was at the airport with her husband and twin 3-year-old daughters waiting to board a plane when the bombs went off.

Belgian law student Leopold Hecht was killed at the metro station in the Brussels attack, his school, Universite Saint-Louis Bruxelles, said in a statement.

The backdrop

“The Belgians have been sitting on a ticking time bomb,” one U.S. counterterrorism official said.

U.S. intelligence officials say they weren’t surprised that Brussels was attacked, because there have been general concerns about terror threats, particularly after recent raids and the arrest of Abdeslam last week.

Belgium has been a top concern for counterterrorism officials for years because of the large number of Belgian foreign fighters who have traveled to join ISIS and other terror groups in Syria and Iraq — more per capita than any other European Union country.

Many have been returning.

Last Friday, after more than four months on the run, Abdeslam was captured after being wounded in a gunfight with Belgian police in Molenbeek. Days later, Belgium and French authorities warned of more attacks.

After Tuesday’s attacks, Belgian officials raised the country’s terror threat warning to its highest level.

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