BRUSSELS » Belgians began three days of mourning Wednesday for the victims of the Brussels airport and subway bombings, and the country remained on high alert as authorities hunted for one of the suspected attackers seen on surveillance video with two others who blew themselves up.

Several people who may be linked to the attacks were still on the loose and the country’s threat alert remained at its highest level, meaning there was danger of an imminent attack, said Paul Van Tigchelt, head of Belgium’s terrorism threat body. The attacks killed 34 people, including three suicide bombers, and injured 270 others, authorities said.

As government offices, schools and residents held a moment of silence Wednesday morning to honor the dead, the mood was defiance mixed with anxiety that others involved in the attacks are still at large.

Belgian prosecutor Federal Prosecutor Frederic Van Leeuw identified two of the Brussels attackers as brothers — Ibrahim El Bakraoui, a suicide bomber at the airport, and Khalid El Bakraoui, who targeted the subway.

Investigators raided the Brussels neighborhood of Schaerbeek after the attacks and found a computer in a trash can on the street including a note from Ibrahim El Bakraoui saying he felt increasingly unsafe and feared landing in prison.

A taxi driver who took Ibrahim El Bakraoui and two others to the airport led investigators to an apartment where they found 15 kilograms (33 pounds) of TATP explosives, along with nails and other materials used to make bombs, Van Leeuw said.

He said authorities do not know the identities of two other people pictured with El Bakraoui in a surveillance photo from the airport that police are circulating.

Two were suicide bombers, the prosecutor said; the other was a man in a white jacket and black cap who fled before the bombs went off, leaving behind a bag full of explosives. That bag later blew up, but no one was injured.

The Islamic State group, which was behind the Paris attacks, has also claimed responsibility for the Brussels bombings.

Belgian state broadcaster RTBF, citing sources it did not identify, said Khalid El Bakraoui had rented an apartment that was raided last week in an operation that led authorities to top Paris attacks suspect Salah Abdeslam.

Abdeslam was arrested Friday in the Brussels neighborhood where he grew up, a rough place with links to several of the attackers who targeted a Paris stadium, rock concert and cafes on Nov. 13. Those attacks killed 130 people.

A Belgian official working on the investigation told the AP that it is a “plausible hypothesis” that Abdeslam was part of the cell linked to the Brussels attack. The official spoke on condition of anonymity because he was not authorized to publicly discuss an ongoing investigation.

Authorities are also still looking for a suspected accomplice of Abdeslam, Najim Laachraoui, whom they have been searching for since last week. It’s not clear if he has any connection to the Brussels attack.

Laachraoui is believed to have made the suicide vests used in the Paris attacks, a French police official told The Associated Press, adding that Laachraoui’s DNA was found on all of the vests as well as in a Brussels apartment where they were made. The official spoke on condition of anonymity because he was not authorized to discuss an ongoing investigation.

French and Belgian authorities have said in recent days that the network behind the Paris attacks was much larger than initially thought — and developments this week suggest the same group could have staged both the Paris and Brussels attacks.

The airport and several Brussels metro stations remained closed Wednesday, and authorities said the airport would remain closed at least through Thursday, forcing the cancellation of 600 flights each day. Security forces stood guard around the neighborhood housing the headquarters of European Union institutions, as nervous Brussels residents began returning to school and work under a misty rain.

In the afternoon, thousands of people gathered at Place de la Bourse in the center of downtown Brussels — including dozens of students chanting “stop the war” — in solidarity with those killed.

“In Belgium, it’s not every day that we show solidarity politically,” said Fanny Nicaise, 24. She came out with some friends just to see and be with others. “It’s important that you aren’t alone in your sadness.”

Belgians paid homage and lit candles, the mood almost buoyant as people wrote on the ground with big sticks of chalk, drawing peace signs and hearts.

As befits an international city like Brussels, the foreign minister said the dead collectively held at least 40 nationalities.

“It’s a war that terrorism has declared not only on France and on Europe, but on the world,” French Prime Minister Manuel Valls said Wednesday on Europe-1 radio. Valls, who planned to visit Brussels later Wednesday, urged tougher controls of the EU’s external borders.

“We must be able to face the extension of radical Islamism … that spreads in some of our neighborhoods and perverts our youth,” he said. The Paris attackers were mainly French and Belgian citizens of North African descent, some from neighborhoods that struggle with discrimination, unemployment and alienation.

In its claim of responsibility, the Islamic State group said its members detonated suicide vests both at the airport and in the subway, where many passengers fled to safety down dark tunnels filled with hazy smoke from the explosion. IS warned of further attacks, issuing a statement promising “dark days” for countries taking part in the U.S.-led anti-IS coalition in Syria and Iraq.

European security officials have been bracing for a major attack for weeks and had warned that IS was actively preparing to strike.

Valls said Wednesday that big events, be they sports or cultural, must not be put on hold for fear of attacks. He said that includes the Euro2016 soccer tournament, a monthlong event being held in France that starts in June. Meanwhile, the Belgian football federation announced that it was calling off an international soccer friendly match against Portugal next week because of the attacks.

Contributing to this report were Associated Press writers John-Thor Dahlburg, Raphael Satter, Danica Kirka and Angela Charlton in Brussels, and Lori Hinnant and Elaine Ganley in Paris.