BRUSSELS — Belgian authorities searched toay for a man pictured at the Brussels airport with two apparent suicide bombers, amid growing suggestions that the bombings of the Brussels airport and subway were the work of the same Islamic State cell that attacked Paris last year.

Several people who may be linked to the Brussels 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.

Belgium began three days of mourning, and govern- ment offices, schools and residents held a moment of silence to honor the dead, marking the moment in a mood of defiance mixed with anxiety that others involved in the attacks are still be 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.

During a raid of the apartment where the brothers had stayed, investigators also found 15 kilograms of TATP explosives, nails, and other material for making explosives, the prosecutor said.

Van Leeuw said authorities do not know the identities of two other people pictured with Ibrahim 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, authorities said. That bag later blew up, but no one was injured.