Same ISIS cell behind Brussels and Paris, but who's left?

Last Updated Mar 25, 2016 10:31 AM EDT

BRUSSELS -- Belgian authorities have formally linked the Brussels terror attacks with those carried out in Paris last November, but there is still confusion over whether they have dismantled, or merely scattered a terrorist cell, reports CBS News correspondent Allen Pizzey.

Raids overnight in Belgium led to at least six arrests linked to last Tuesday's airport and subway bombings. One of the operations Thursday night was in the Schaerbeek neighborhood where the police earlier this week found explosives and bomb-making material used by the Brussels attackers.

On Friday, the mayor of Schaerbeek confirmed that a large police raid in the district was linked to the investigation into the Brussels suicide bombings and a new arrest in the Paris area. State broadcaster RTBF quoted mayor Bernard Clerfayt as saying one person was "neutralized" in the operation. He did not say whether that meant the person was arrested or wounded.

A U.S. law enforcement official confirmed to CBS News on Friday that Belgian authorities want to find and question a man named Abdelhamid Kassoul, to determine whether he was the third, still officially-unidentified man seen at the airport with bombers Ibrahim El Bakraoui and Najim Laachraoui.

According to the source, Belgian authorities said immediately after the airport bombing, Kassoul was believed to have boarded a shuttle to another area of the airport and has not been seen since. He is now among a handful of people Belgian investigators want to interview in relation to the ongoing investigation into the airport and subway attacks.

The police are also still looking for a possible fifth suspect; a man seen entering the Brussels Metro system with suicide bomber Khalid El Bakraoui.

Meanwhile, the terrorist threat level alert has been lowered from four -- the highest -- to three, although authorities said the situation remained grave and another attack is "likely and possible."

Another raid, in France, led to the arrest of a man connected to the mastermind of last year's Paris attacks.

Two of the suicide bombers -- the Bakraoui brothers -- were on U.S. law enforcement radar. Both had criminal records in Belgium. Khalid was subject to an international arrest warrant on terrorism charges and Ibrahim had been deported from Turkey.

Another suicide bomber, Laachraoui, was a suspected bomb-maker for the attacks in Paris last November.

The alleged logistics chief of those attacks was caught before the Brussels bombings, but Salah Abdeslam's lawyer, Sven Mary, told CBS News the police had only interrogated him once.

"You know he was interviewed from the first time on Saturday. He didn't see any police officers before the attacks. So he saw police officers after the attacks," Mary said.

Abdeslam insisted he had not been aware of the plans for the Brussels attacks, even though he shared hideouts with the perpetrators.

According to French media reports, Abdeslam said he wore a suicide vest for the Paris attacks, but could not blow himself up.

As for reassuring the nervous Belgian public, the prime minister said that there would be "no grey areas" in the investigation and hunt for more terrorists.

The police and security agencies have admitted to failures in coordination and intelligence sharing and pledged to fix them. The open question is whether they can do it in time to prevent more attacks.