A victim receives first aid from rescue personnel at the Maalbeek metro station on Tuesday | Emmanuel Dunand/AFP via Getty Images ISIL strikes Brussels, police launch manhunt ‘What we feared has happened,’ said Belgian PM Charles Michel.

Terrorists struck at the heart of Europe on Tuesday, killing at least 31 people with bombs at Brussels airport and a subway train in the EU quarter.

Coming four months after the Paris attacks, the mayhem in Brussels once again exposed the Continent's vulnerability to terrorism, and will likely have far-reaching political consequences for Belgium and its European allies. It is sure to add new urgency to the debates over open borders, the balance between civil liberties and security, the Syria conflict, the migration crisis and the best ways to address the radicalization of young Muslim men born in the EU.

The rush-hour strikes on Zaventem airport and the metro system appeared calculated to ridicule Belgium's claim that it had dealt a blow to ISIL with last Friday's arrest in Brussels of Salah Abdeslam, who allegedly handled the logistics for the attackers in Paris on November 13.

Abdeslam was supposed to take part in Tuesday's attack, said one Belgian official. His fingerprints were found in an apartment in Brussels last week, where the official said police also found detonators that were probably meant to be used in the bombings. Belgium's foreign minister had warned after Abdeslam's capture that he was preparing further attacks.

Two suicide bombers carried out the airport attack and one was responsible for the subway blast, officials said, but at least one of the airport attackers appeared to have escaped. Belgian police launched a manhunt for a suspect who was photographed on their website wearing a hat and glasses, whom they said was "suspected of being implicated" in the attacks.

They had earlier released an image of him with two other men, all pushing luggage carts through the terminal minutes before the explosions. Two of them had their left hands covered, possibly concealing detonators, local media reported.

The Islamic State of Iraq and the Levant claimed responsibility for the Brussels bombings, announcing via its Amaq media outlet that fighters carried out attacks against “a country participating in the international coalition against the Islamic State.”

The public, pre-security area of the international airport terminal and the underground train crowded with commuters pulling into Maalbeek station presented "soft" targets to the bombers. The EU and Belgian capital has been on security alert level 3 — the second highest — since Paris, with military patrols on the streets, metro platforms and at large public events.

"What we feared has happened," said Prime Minister Charles Michel. "Belgium was hit by a blind, violent and cowardly attack."

His government has come under criticism for allowing violent Islamic radicalism to flourish in the middle of the capital in the Molenbeek quarter where Belgian-born Abdeslam's family lived — and where he was arrested last week when police raided a flat and shot the 26-year-old in the leg to immobilize him.

'Screaming in Arabic'

Within minutes of the morning strikes, the Belgian government raised its threat level to four and effectively closed off the capital, suspending airline flights and all public transport in Brussels throughout Tuesday. This brought back memories of the "lockdown" in November, when police searched the entire city in vain for the Paris suspects, after investigators traced some of them back to Molenbeek.

The government declared three days of national mourning for the victims.

The first explosions struck shortly after 8am at Zaventem airport. Smoke poured from a damaged terminal building as panicked passengers including many injured people rushed away from the site, some still carrying luggage.

One witness said he heard a man shouting in Arabic before the first blast.

"I heard 'bang', and then a man screaming in Arabic," said Alphonse Youla, 40, who works for a security firm. "There was a couple withdrawing cash in front of me. The woman exploded and lost both legs. Her husband lost one leg and so did a police officer."

"The windows of shops were blown out and there was blood everywhere," he added.

As hospitals throughout Brussels dealt with the dead and injured, officials said at least 11 people were killed and 100 wounded in the attack on Zaventem. The preliminary death toll in the subway bombing, which happened about an hour later, was 21 people with about 130 injured.

The blast hit the front carriages of a train pulling into a station that serves the European Union's main institutions, including the Commission, Council and Parliament. Smoke billowed from the station entrance and some of the dead and injured were taken to a nearby hotel, where the lobby was full of people covered in blood, according to a POLITICO reporter on the scene.

"I was on the metro train pulling into the station when there was a massive explosion," said one commuter, who asked not to be named. "Lights went out, smoke everywhere, sounds of multiple explosions, everyone dropped to the floor. Thankfully I managed to escape by prying the car door open with some other passengers and running for the exit."

A police bomb squad then carried out another controlled blast at nearby Arts-Loi metro station, by which time the entire metro system had been evacuated.

In a raid on a flat in the Brussels neighborhood of Schaerbeek later on Tuesday, police found an explosive device filled with nails as well as chemicals and an ISL flag.

Dark morning in Brussels

Belgium had been on the alert for fresh attacks following last week's arrest of Abdeslam, especially since at least two suspects connected to the Paris plot -- including Najim Laachraoui, who is believed to be a bomb-maker -- were still on the run.

Asked if the attacks could be linked to Abdeslam's arrest last Friday, Michel said: "We don't have any information on that."

However, neighboring Germany's interior minister, Thomas de Maizière, said it was possible the attacks were sparked by the arrests, adding that the bombers were not just targeting Belgium but "everyone who is part of the EU.”

Belgian authorities opened an emergency number for those concerned about relatives: +32 2 753 7300. Flags flew at half-staff in the European quarter and messages of condolence poured in from world leaders including German Chancellor Angela Merkel, French President François Hollande, U.S. President Barack Obama and Russia's Vladimir Putin.

"Dark morning here in Brussels," tweeted Jens Stoltenberg, the head of NATO which is also headquartered in Brussels. "My thoughts are with those affected and the Belgian people. We stand together with our ally Belgium."

“France and Belgium are connected by terror,” said French President François Hollande.

Chris Spillane, Anca Gurzu, Craig Winneker and Silvia Sciorilli Borrelli contributed reporting for this article.