Brussels, Belgium (CNN) Police detained six people in raids Thursday night as investigators raced to uncover the network behind this week's terror attacks in the Belgian capital .

The Belgian federal prosecutor's office didn't provide details about who had been detained in the Brussels raids, why they had been apprehended or whether they will face charges.

"It will be decided tomorrow if these people will remain in custody," the office said in a statement released late Thursday.

Two people were taken into custody in Brussels' Jette neighborhood, one person was detained in a different part of the capital, and three people were in a vehicle in front of the federal prosecutor's office when authorities apprehended them, public broadcaster RTBF reported.

So far, authorities have said they believe five men played a part in Tuesday's bombings in Belgium that killed 31 people and injured 330. Three of the attackers are dead. Two of them could still be on the loose.

Investigators are combing over evidence from surveillance footage and the explosives stash they seized from an apparent hideaway in a suburb.

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

Sweeps where investigators detain people first and ask questions later are likely to become an increasingly common tactic, CNN national security analyst Juliette Kayyem said.

"There will be lots more of them," she said. "They are going to be what's called overbroad. They are going to just try to find people or evidence that may stop the next terrorism attack, and they will figure out who they have under custody."

Surveillance footage shows man with bag

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Khalid El Bakraoui, one of the terrorists who bombed a train near the Maelbeek metro station, is dead. Authorities believe a second unidentified person was also involved in that attack, a senior Belgian security source told CNN. But investigators don't know where that suspect is -- or whether he's dead or alive.

Surveillance footage shows the 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.

Authorities have released a grainy image of another suspect who they believe is on the run.

That man, they say, shown in photographs wearing a black hat, was one of three attackers at Brussels Airport. Authorities say he planted a bomb at the airport and left. The other two men in the photographs are believed to be the suicide bombers.

Fair to ask whether 'we missed the chance'

Did Belgian authorities miss a chance to stop at least one of the suspects involved in the attacks?

Photos: Terror attacks in Brussels Photos: Terror attacks in Brussels An injured woman leaves the airport in Brussels, Belgium, after two explosions rocked the facility on Tuesday, March 22. There was also an explosion at a subway station in the city. Hide Caption 1 of 22 Photos: Terror attacks in Brussels Windows are blown out after the deadly attack at the airport. Hide Caption 2 of 22 Photos: Terror attacks in Brussels A police officer directs passengers in a smoke-filled airport terminal. Hide Caption 3 of 22 Photos: Terror attacks in Brussels Two wounded women are seen in the airport. Hide Caption 4 of 22 Photos: Terror attacks in Brussels This photo from inside the airport was shared by Jef Versele on Facebook. Hide Caption 5 of 22 Photos: Terror attacks in Brussels Subway passengers walk along the tracks following a blast at the Maelbeek metro station. Hide Caption 6 of 22 Photos: Terror attacks in Brussels A private security guard helps a wounded woman outside the Maelbeek metro station. Hide Caption 7 of 22 Photos: Terror attacks in Brussels Wounded people are treated outside the subway station. Hide Caption 8 of 22 Photos: Terror attacks in Brussels Rescue teams evacuate the subway station. Hide Caption 9 of 22 Photos: Terror attacks in Brussels Police officers guard the area around the subway station. Hide Caption 10 of 22 Photos: Terror attacks in Brussels People react as they walk away from the Brussels airport. Hide Caption 11 of 22 Photos: Terror attacks in Brussels A man with blood stains on his sweater leaves the airport. Hide Caption 12 of 22 Photos: Terror attacks in Brussels Passengers leave the airport after the attack. Hide Caption 13 of 22 Photos: Terror attacks in Brussels A young girl looks out of the window of a bus after airport evacuations. Hide Caption 14 of 22 Photos: Terror attacks in Brussels People stand near the airport after evacuations. Hide Caption 15 of 22 Photos: Terror attacks in Brussels Passengers gather outside the airport. Hide Caption 16 of 22 Photos: Terror attacks in Brussels Police officers stand guard near the Maelbeek metro station. Hide Caption 17 of 22 Photos: Terror attacks in Brussels A police helicopter flies above the area near the subway station. Hide Caption 18 of 22 Photos: Terror attacks in Brussels People embrace outside the Brussels airport. Hide Caption 19 of 22 Photos: Terror attacks in Brussels People are led away from the airport after the attacks. Hide Caption 20 of 22 Photos: Terror attacks in Brussels A victim receives first aid near the Maelbeek metro station. Hide Caption 21 of 22 Photos: Terror attacks in Brussels Ambulances arrive at the airport. Hide Caption 22 of 22

Interior Minister Jan Jambon suggested that's a legitimate question, after Turkish officials revealed that Belgian authorities had been flagged back in 2015 when Turkey captured Ibrahim El Bakraoui near the Syrian border. El Bakraoui was deported to the Netherlands and set free.

Bakraoui had been sentenced to nine years in prison in Belgium back in 2010 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.

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from the Brussels neighborhood where he grew up before his arrest. Belgian authorities are facing criticism for taking so long to track down Paris attacks suspect Salah Abdeslam, who was captured last week after four months on the run. Abdeslam hid in plain sight just blocks from the Brussels neighborhood where he grew up before his arrest.

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

Bomber brothers' homes searched

Authorities looked Wednesday at the Brussels homes of the Bakraoui brothers. Those two searches "were not conclusive," the federal prosecutor's office said.

Homes were searched Thursday in several areas in and around the city, officials said.

One operation in the neighborhood of Schaerbeek stretched for hours into Friday morning. Investigators sealed off streets for several blocks. It was not immediately clear why such a large area had been cordoned.

Masked teams in hazmat gear could be seen exiting a building and heading toward a police van.

As investigations continue, a larger question looms: What could happen next?

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.

Photos: World reacts to Brussels bombings Photos: World reacts to Brussels bombings Airport workers and their relatives in Brussels, Belgium, hold a candlelight vigil Wednesday, March 23, to pay tribute to those who were killed in terrorist attacks the day before. On Tuesday, explosions rocked the city's airport and a subway station. Hide Caption 1 of 23 Photos: World reacts to Brussels bombings Belgian national flags are projected onto the National Gallery in London's Trafalgar Square on March 23. Hide Caption 2 of 23 Photos: World reacts to Brussels bombings A woman in Brussels pauses after people observed a minute of silence at the Place de la Bourse on March 23. Hide Caption 3 of 23 Photos: World reacts to Brussels bombings Teresa Mancheno, a maintenance worker at Newark Liberty International Airport, attends a vigil in Newark, New Jersey, on March 23. Hide Caption 4 of 23 Photos: World reacts to Brussels bombings European Union flags fly at half-staff outside the European Commission in Brussels on March 23. Hide Caption 5 of 23 Photos: World reacts to Brussels bombings A young girl lights a candle at the Place de la Bourse in Brussels on Tuesday, March 22. Hide Caption 6 of 23 Photos: World reacts to Brussels bombings An image of the Belgian flag is displayed on the Trevi Fountain in Rome on March 22. Hide Caption 7 of 23 Photos: World reacts to Brussels bombings The Eiffel Tower is lit up with the colors of the Belgian flag on March 22. Hide Caption 8 of 23 Photos: World reacts to Brussels bombings Servicemen with Azov, a Ukrainian volunteer battalion, hold torches during a tribute ceremony at the Belgian Embassy in Kiev, Ukraine, on March 22. Hide Caption 9 of 23 Photos: World reacts to Brussels bombings U.S. President Barack Obama and his family observe a moment of silence as they attend a baseball game in Havana, Cuba, with Cuban President Raul Castro, right, on March 22. Hide Caption 10 of 23 Photos: World reacts to Brussels bombings A woman reads messages written on the ground at Brussels' Place de la Bourse on March 22. Hide Caption 11 of 23 Photos: World reacts to Brussels bombings A New York City church holds Mass for victims of the Brussels attacks on March 22. Hide Caption 12 of 23 Photos: World reacts to Brussels bombings A man places flowers outside the Belgian Embassy in Moscow on March 22. Hide Caption 13 of 23 Photos: World reacts to Brussels bombings A man looks at flowers and messages outside the stock exchange in Brussels on March 22. Hide Caption 14 of 23 Photos: World reacts to Brussels bombings Democratic presidential candidate Hillary Clinton, center, stands for a moment of silence during a roundtable with tribal leaders in Puyallup, Washington, on March 22. Hide Caption 15 of 23 Photos: World reacts to Brussels bombings A woman leaves a bouquet of flowers at the base of the Belgium and European Union flags, which were flying at half-staff March 22 at the Belgian Embassy in Washington. Hide Caption 16 of 23 Photos: World reacts to Brussels bombings Activists in Multan, Pakistan, condemn the Brussels attack on March 22. Hide Caption 17 of 23 Photos: World reacts to Brussels bombings A woman lays flowers at the steps of the Belgian Embassy in Berlin on March 22. Hide Caption 18 of 23 Photos: World reacts to Brussels bombings A boy at a makeshift migrant camp shows support for the victims near the village of Idomeni, Greece, on March 22. Hide Caption 19 of 23 Photos: World reacts to Brussels bombings Mayors of Istanbul districts walk with consuls from various countries, including Belgium, during a protest condemning terrorism on March 22. Hide Caption 20 of 23 Photos: World reacts to Brussels bombings People in Turin, Italy, take part in a rally to remember the victims on March 22. Hide Caption 21 of 23 Photos: World reacts to Brussels bombings France's Parliament observes a minute of silence on March 22. Hide Caption 22 of 23 Photos: World reacts to Brussels bombings Members of Quebec's National Assembly have a moment of silence on March 22. Hide Caption 23 of 23

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.