Belgian prosecutors on Saturday charged four people rounded up in the past 24 hours with being part of a terrorist organization, adding it was still unsure whether Mohamed Abrini was the so-called "man in the hat" involved in the Brussels terror attacks.

Prosecutors said in a statement "it was not possible yet to confirm that Mohamed Abrini indeed was the third suspect" seen at the Brussels airport wearing a hat and glasses alongside the two suicide bombers, Ibrahim el-Bakraoui and Najim Laachraoui.

The "man in the hat" is suspected to have fled without blowing up his bomb. It was later found abandoned at the airport.

Abrini had been on Europe's most wanted list since CCTV footage at a gas station in northern France showed him with Paris suspect Salah Abdeslam two days before the deadly November attacks, which killed 130 people.

Abdeslam was arrested two weeks ago, four days before "Islamic State" suicide bombers targeted the Brussels airport and a downtown metro stop on March 22, killing 32 people.

Prosecutors on Saturday also leveled terrorism charges against Swedish national Osman K.

He is believed to be Osman Krayem of the Swedish city of Malmo, who had gone to fight in Syria. Krayem is alleged to have bought the bags used in the Brussels bombings and was seen at the Maalbeek metro station in downtown Brussels before suicide bomber Khalid el-Bakraoui blew himself up.

Also charged for their role in the Brussels attacks were Herve B. M., a Rwandan national, and Bilal E. M.

Two other suspects arrested since Friday were released after being questioned.

The charges follow a string of arrests that have shed light on the links between the IS terror network believed to be behind the Paris and Brussels attacks. Several of the attackers are believed to have spent time in Syria with the group, which has vowed further attacks in Europe.

"There are perhaps other cells that are still active on our territory," Belgian Interior Minister Jan Jambon cautioned on RTL television on Saturday.

cw/rc (AFP, AP, dpa, Reuters)