DEVELOPING: Explosions and gunfire were heard Friday during police raids in the Brussels neighborhood where the suicide bomber suspected of making the explosives used in Tuesday’s terror attacks assembled the deadly devices.

At least one unidentified person was detained as police conducted multiple operations in Schaerbeek, according to state broadcaster RTBF. Witnesses reported hearing three explosions and gunfire in the operation, which involved as many as 50 police and ended around 3:30 p.m. local time.

The raids were part of an inteational effort to roll up a terror cell believed responsible for coordinated attacks carried out Tuesday in Zaventem Airport near Brussels and at a metro station in the city, as well as the terror attack that killed 130 in Paris Nov. 13.

What is known of the men suspected of direct involvement in Tuesday’s attacks:

  • Ibrahim El Bakraoui, one of two brothers involved, is believed to be one of two suicide bombers who died at the airport.
  • Najim Laachraoui, an ISIS explosives expert believed to have built the bombs used in both the Paris and Brussels attacks, was the other suicide bomber who died at the airport. He and Ibrahim El Bakraoui are believed to be the men seen in a surveillance photo pushing luggage carts and wearing solitary black gloves that may have masked detonators.
  • A mystery man dressed in white, wearing a dark hat and possibly a disguise who has not been identified was also seen pushing a cart in the surveillance photo. He is believed to have placed a bomb at the airport and fled the scene. Authorities are looking for him.
  • Khalid El Bakraoui, the brother of Ibrahim El Bakraoui, is believed to have died in a suicide blast at the Maelbeek Metro station 79 minutes after the airport attack.
  • A second man seen with Khalid El Bakraoui and carrying a large bag at the Metro station is believed to have been an accomplice. Police are trying to determine if he is a suspect who was arrested Thursday night. His identity has not been revealed.

Schaerbeek Mayor Beard Clerfayt told RTBF one person had been "neutralized" in the operation Friday, but did not elaborate. Clerfayt did confirm the raid was linked to Tuesday's attacks.

Prime Minister Charles Michel skipped a wreath-laying ceremony at the Brussels airport with U.S. Secretary of State John Kerry because of the ongoing police operation.

The developments followed a night of police activity in the Belgian capital amid the continuing dragnet for at least one and possibly two bombers believed to have survived the Brussels attacks. Three other bombers, including explosives expert Najim Laachraoui, who is believed to also have made the bombs used in the Paris massacre, died in the attacks, according to authorities.

The Belgium attacks killed 31 and injured at least 270. ISIS has claimed responsibility.

Earlier this week, police found a large stash of explosives in an apartment in Schaerbeek linked to Laachraoui. it was unclear whether the explosions heard early Friday afteoon were controlled police detonations or something else.

Known to have been killed in the two attacks are Ibrahim El Bakraoui and Laachraoui, who blew themselves up at the airport, killing 11; and El Bakraoui's brother, Khalid El Bakraoui, who blew himself up in the metro station and was seen with an accomplice.

Belgian investigators told local media they believe a man arrested Thursday could be the second man seen in the metro attack, a person familiar with the investigation said.

"We have strong indications that this is the suspect who was hunted for the last couple of days. The identification is still ongoing," he said, referring to man caught on CCTV entering Maelbeek station with presumed bomber Khalid El Bakraoui.

Another suspected bomber, seen in the airport wearing a dark hat and possibly a disguise, is being sought.

Also on Thursday night, French police conducting anti-terror raids outside Paris nabbed the suspected ringleader of the terror attacks in the French capital. The Associated Press, citing two French officials, identified the suspect as Reda Kriket, 34. He was arrested in Argenteuil, on the northe outskirts of Paris.

In all, seven people have been arrested in Belgium in coection with the attacks. A source told Reuters that among those arrested in Brussels on Thursday were three people "stupidly hanging around" near the federal prosecutors office, the heavily guarded center of the investigation effort. The source said investigators were still trying to determine if they were part of the terror cell.