Belgian federal prosecutors on Monday said three more people have been ordered held on charges of participating in terrorist activities.

Prosecutors did not immediately release details on the alleged terrorist acts or whether they were linked to last week's suicide bombings at Brussels Airport and in the Brussels subway that killed at least 31 people and injured some 270 others.

The three charged by an investigating magistrate - identified as Yassine A., Mohamed B. and Aboubaker O. by the federal prosecutors office - were among four people detained during 13 police raids Sunday in Brussels and the northern cities of Mechelen and Duffel.

A fourth person detained was released without charge, the prosecutors' statement said.

On March 22, two suicide bombers blew themselves up at Brussels Airport and a third did the same in a Brussels subway train.

The attacks caused great damage to the airport's departure area. On Tuesday, one week after the bombings, Brussels Airport will test its capacity to partially resume passenger service, an airport official said Monday. But she said it's too early to say when passenger service might actually resume.

Florence Muls, the airport's external communications manager, said 800 staff members on Tuesday will test temporary infrastructure and new arrangements designed for passenger check-in. Firefighters and the Belgian government must approve the new system, Muls said, before Brussels Airport can resume handling passenger traffic.

Before the bombings, Brussels Airport served some 600 flights a day and 23.5 million passengers per year.

Meanwhile, the website of Belgium's Federal Police on Monday began carrying a 32-second video of a mysterious man in a hat suspected of having taking part in the March 22 bombing of Brussels Airport.

"The police are seeking to identify this man," the site says.

The implication is that the suspected accomplice of the two airport suicide bombers could still be at large. Last week, Belgian media reported that Faycal Cheffou - reported to be the man identified by authorities as Faycal C. who is in custody and has been charged with terror offenses - had been identified as the suspect who escaped from Brussels Airport following the bombing.

The video shows the man, wearing glasses and a white jacket, wheeling a baggage cart through the airport along with the two men identified by Belgian authorities as the bombers.

Death toll rises

The Belgian health minister says four of those wounded in the suicide bombings last week have died in the hospital, bringing the number of victims of the bombings to 35.

The minister, Maggie De Block, made the announcement on her Twitter account Monday morning.

She posted: "Four patients deceased in hospital. Medical teams did all possible. Total victims: 35. Courage to all the families."

Suicide bombers hit Brussels Airport and a city subway train on March 22 in an attack claimed by the Islamic State group.

Airport may fully open soon

A week after devastating suicide bomb attacks, Brussels Airport will test its capacity to partially resume passenger service. But it's too early to say when service might actually resume, an airport official said Monday.

Florence Muls, the airport's external communications manager, said 800 staff members on Tuesday will test temporary infrastructure and new arrangements designed for passenger check-in.

The Belgian government must approve the new system, Muls said, before Brussels Airport can resume handling passenger traffic.