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
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?
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.
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.
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.