BRUSSELS — Belgian authorities searched Wednesday 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 government 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.

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. Belgian newspaper DH initially reported he might be the man in the white jacket at the Brussels airport, but later removed that report from its website.

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.