BRUSSELS-- Belgian authorities have admitted to serious mistakes, but they claim they are making progress and have inflicted damage against theterrorist network suspected of carrying out both the Brussels and Paris attacks.

CBS News correspondent Allen Pizzey reports, however, that exactly how much damage European law enforcement agencies have inflicted on that European network of apparent ISIS operatives remains an open -- and vexing -- question.

Security was still tight Tuesday morning in the Belgian capital and the airport was still closed. As one official conceded, Brussels will never return to the so-called "normal" again.

Belgian Federal Police via AP

Belgian police thought they had arrested the now-infamous "man in the black hat" seen on security camera video from Brussels Airport, seemingly accompanying the two other bombers in that attack. They laid charges that included "terrorist murder" against the man, a French journalist, but the suspect had a credible alibi, the police evidence didn't stick.

Their prime suspect was released without charge on Monday.

A series of raids across Europe in the wake of last week's attacks have uncovered parts of a terrorist web more complex than many imagined.

A suspect shot in the leg and arrested at a Brussels tram stop, and another picked up in a raid, are alleged accomplices of a third man arrested in France whom police said was in "an advanced stage of planning" for a fresh attack.

ISIS has claimed responsibility in a new video, and included taunts aimed at European law enforcement.

"We are everywhere," the jihadist jeers in French. "We are capable of striking wherever we want, whoever we want...whenever we want."

And as Pizzey notes, that boast isn't idle; much of the network being uncovered is "home-grown."

"Many, many terrorists that are active today have never set foot... in the Middle East. They became terrorists in jail," terrorism expert Gef Lambrecht told CBS News.

A memorial service for the victims was a reminder to the authorities of how much pain remains, and how many people are still waiting for answers.

The search for the suspects still out there -- including two believed directly related to the Brussels attacks -- is now getting help from the FBI.

Intelligence sources tell CBS News the agency is involved at "the highest level" with Belgian officials and with their counterparts across Europe, providing intelligence and combing through data basess.