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(CNN)Five terrorists -- three dead, two whose fates are unknown.

So far, that's how many people that authorities say played a part in Tuesday's terror attacks in Brussels, Belgium, that killed 31 people and injured 330.

Two were allegedly involved in bombing a train near the Maelbeek metro station. Of them, one has been identified as Khalid El Bakraoui, who died in that suicide attack. And on Thursday, a senior Belgian security source told CNN that authorities believe a second, unnamed person was also involved in that blast. That man's whereabouts -- and whether he's dead or alive -- is unknown.

Surveillance footage shows this man holding a large bag at the station, according to Belgian public broadcaster RTBF. It's not clear if he was among the at least 20 killed in that blast, RTBF said.

Three more people apparently took part in the Brussels Airport attack. At least two of them are dead -- Khalid's brother Ibrahim El Bakraoui and ISIS bomb maker Najim Laachraoui, who also had ties to the November Paris terror attacks. Authorities also have a grainy image of another suspect who they believe is on the run.

And they've uncovered an apparent hideout in a Brussels suburb containing 15 kilograms (33 pounds) of the explosive TATP, screws and other bomb-making materials, Belgian federal prosecutor Frederic Van Leeuw said.

Any clue, any lead, any witness could be critical in the race to stop the next terrorist attack on European soil.

CNN intelligence and security analyst Bob Baer thinks people have reason to worry, given all the munitions authorities have found so far -- and, more so, those they still haven't discovered. He says Britain or France could be hit next.

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"I think they're shocked by the amount of armament and the number of followers and how big the network was connected to Paris and now Brussels," Baer said.

As CNN International Diplomatic Editor Nic Robertson puts it: "The dam has broken''.

Minister: Possible 'we missed the chance'

Are Belgian authorities responsible, at least in part, for this rupture?

Interior Minister Jan Jambon suggested Thursday that's a legitimate question, especially after Turkey's presidency revealed that it had captured Ibrahim El Bakraoui in June 2015 and deported him to Europe, where he was set free.

Bakraoui was the same man who, in October 2010, was sentenced to nine years in prison for opening fire on police officers with a Kalashnikov during a robbery, according to broadcaster RTBF and CNN affiliate RTL. Needless to say, he didn't serve all that time.

"Given the facts, it is justified that ... people ask how it is possible that someone was released early and we missed the chance when he was in Turkey to detain him," said Jambon, whose offer to resign was rebuffed by Prime Minister Charles Michel.

This isn't the only potential embarrassment for Belgian authorities. Another is that Salah Abdeslam -- despite being Europe's most wanted man for four months as the lone surviving Paris attacker -- hid in plain sight just blocks from the Brussels neighborhood where he grew up before his capture last week.

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Baer, the CNN analyst, thinks European authorities haven't done enough to develop a network of sources inside communities that terrorists call home.

"These communities of North African origin are very tight-knit," he said. "It's very difficult for the European services to get inside of them. They were ignored for so many years."

Brussels eyewitness: 'Belgium doesn't deserve this'

The fact that no one gave Abdeslam up is troubling. What may be more worrisome is if there are more young men like him, who sympathize not with Europeans but with ISIS, the terror group that's taken over swaths of territory in Syria and Iraq while claiming attacks elsewhere around the world, including in Brussels and Paris.

"Today, Belgium has been trying to hold back another force of nature: A disaffected, disassociated youth, warped and wrapped in ISIS's corrosive ideology," CNN's Robertson said. "Their numbers have been too great for Belgian counterterrorism efforts to cope."

Bomber brothers' homes searched

Not long ago, Western authorities believed ISIS was focused on taking territory in Syria and Iraq, not lashing out elsewhere. But U.S. officials now think the extremist group has been sending trained militants to Europe for some time.

These men don't necessarily follow orders directly from ISIS headquarters. But they build on what they've learned, as well as a shared philosophy and approach, to develop their own terror cells and hatch their own plots.

How many more ISIS militants are in Europe, poised to attack? That's not clear.

For now, though, the top priority is tracking down the two men linked directly to Tuesday's terror.

One of them, shown in surveillance images wearing a hat and light-colored clothing, is suspected of dropping off a bomb at the Brussels Airport and then leaving.

The three men who are suspected of taking part in the attacks at Belgium's Zaventem Airport.

Authorities looked Wednesday at the Brussels homes of the Bakraoui brothers, though the Belgian federal prosecutor's office says those two searches "were not conclusive."

The same office revealed that Khalid El Bakraoui, the subway bomber, was the subject of an Interpol "red notice" issued on December 11, 2015. He allegedly used false identification to rent a Brussels apartment that served as a hideout for Paris attackers.

Those who stayed there include Abdeslam, who Belgian officials say may have been helping plan new attacks at the time of his capture.

Investigators suspect Abdeslam planned to be part of an attack by the same ISIS cell that lashed out Tuesday in Brussels, a senior Belgian counterterrorism official told CNN's Paul Cruickshank.

What we know about the terror attacks in Belgium

CNN's Christiane Amanpour, Nima Elbagir, Paul Cruickshank, Evan Perez, Margot Haddad, Holly Yan, Anna-Maja Rappard and Mick Krever contributed to this report.