The unnamed person was taken into custody in Anderlecht, one of the districts in the capital region of Brussels. Brussels is where, on Tuesday, terrorists killed at least 30 people and wounded another 230 in horrific bombings at the city's international airport and Maelbeek metro station.
Two suicide bombers -- identified as brothers Khalid and Brahim El Bakraoui -- are believed to have killed themselves in the airport attacks, a senior Belgian security source told CNN.
Both brothers were known to police, but for organized crime, not for acts of terrorism, according to RTBF.
The brothers could be two of the three men seen in an airport security image released Tuesday by authorities.
Belgian federal Prosecutor Frederic Van Leeuw said the two in dark-colored clothing were apparent suicide bombers.
But investigators believe the third man pictured -- wearing light-colored clothing and a hat -- planted a bomb at the airport, then left in a move that appeared to be planned, two U.S. officials said.
Where did he go? What was he planning to do next? And is he know in custody?
Those are some of the many looming questions facing investigators Wednesday.
Expert: Identifying brothers could spur investigators
Khalid El Bakraoui rented an apartment in Brussels that was raided last week, and both are suspected of having ties to the terror attacks in Paris in November, the source said.
While Belgian officials say both brothers were suicide bombers, a U.S. official briefed earlier on preliminary evidence from the investigation says authorities are looking at the possibility that one of the airport explosions may have been caused by a bomb inside a suitcase and the other was a suicide bombing.
But identifying the brothers should help spring the investigation forward, says Cedric Leighton, a CNN military analyst and the former deputy director for the Joint Chiefs of Staff.
"You can start basically peeling back the onion," he told CNN. "Hopefully what it will do is it will speed up the process by which they can actually look at all of the different elements of this and possibly roll up some more suspects."
The hunt is on for a third remaining suspect -- a man in light-colored clothing who appeared next to the El Bakraoui brothers at the airport in security footage.
Three suspects, two explosions and a taxi driver
"It's Salah Abdeslam all over again," Dirk Coosemans, a reporter at Belgian newspaper Het Nieuwsblad, told CNN, referring to a key Paris attack suspect who was arrested last week. "This one was there to be a suicide terrorist, and he didn't do it."
Belgian media also reported a Kalashnikov assault rifle was found in the departure hall of the Zaventem airport.
Fortunately, a taxi driver called police shortly after the security image was released and said he believed that he drove the trio to the airport.
The driver told authorities that his passengers would not allow him to unload the suitcases from the cab. He also led investigators to where he picked the three of them up.
Schaerbeek
That information prompted authorities to raid a residence after the attacks, the officials said.
Investigators found a nail bomb, chemical products and an ISIS flag during a house search in the northeast Brussels neighborhood of Schaerbeek, Belgium's federal prosecutor said in a statement.
Forensic teams are now scouring an apartment building in that neighborhood and have been seen carrying out bags of evidence, according to CNN's Frederik Pleitgen, who was reporting from just outside the building.
Their work continued into the night.
Putting the pieces back together
Determining what type of explosives were used will be crucial, according to CNN National Security Analyst Peter Bergen.
French prosecutors have said that the bombs used in the November Paris attacks were made from triacetone triperoxide, or TATP.
If the same type of bombs were used in Paris and Brussels, that would be a key clue linking the two attacks.
"Such bombs have been a signature of jihadist terrorists in the West for more than a decade because the materials are so easy to acquire, unlike military-grade explosives, which are tightly controlled in much of the West," Bergen said.
TATP-based bombs require technical know-how and bulk purchases of hydrogen peroxide or hair bleach. That helps authorities narrow down potential bomb-making suspects, because making the explosives can sometimes bleach hair. So authorities can identify bomb-makers in part by recognizing unusually bleached hair or asking sellers to report any suspiciously large purchases of hydrogen peroxide.
Dearth of Maelbeek information
While authorities have been able to move quickly on intelligence from the airport attacks, very little has been publicly revealed about the bombing at the Maelbeek metro station.
Coosemans, the Het Nieuwsblad reporter, says that's because there isn't as much surveillance there, compared with the airport.
"We just know less about the Maelbeek attack because we don't have pictures there," he told CNN. "The police know less about Maelbeek."
Unraveling the network
Two senior U.S. officials told CNN they believe the Belgium attack is tied to the same network as terror suspect Abdeslam. ISIS has claimed responsibility for the attack.
And the "working assumption" in Belgium is that the attackers came from the network behind the Paris attack, Belgian security sources said. However, they cautioned it is very early in the latest investigation.
Intelligence sharing will be very important, says Steve Moore, a CNN law enforcement contributor.
"They obviously have some information. They don't know if they're looking at one cell or a series of cells. And so now it's time to get all around at the same table and exchange information," he said. "If you can get them all to use the same currency, I cannot believe that you can't get them all to share intelligence."
