It's one key piece of evidence authorities are looking at as they search for suspects after two explosions at the Brussels airport and another at a busy subway station in the Belgian capital Tuesday killed at least 30 people and wounded 230 others.
ISIS claimed responsibility for the coordinated attacks, but authorities said it's too soon to say for sure whether the terror group was behind the blasts.
Two of the men, wearing black in surveillance images, are believed to be suicide bombers who died in the explosions in the airport's departure lounge.
But investigators believe the one in light-colored clothing planted a bomb at the airport, then left. Authorities called him a wanted man and asked for the public's help tracking him down.
"The third man left a bomb in the airport, but it didn't explode. ... And we are now looking for this guy," Belgium's Interior Minister Jan Jambon said.
A photograph released by investigators shows the three suspects side-by-side.
Federal Prosecutor Frederic Van Leeuw said the two men wearing black in the photograph were likely the suicide attackers.
Video shows the men exiting a taxi and moving through the airport, according to two U.S. officials. The man dressed in white left the airport after accompanying the other two, they said -- a move the officials said appeared to be planned.
Taxi driver tip sparks raid
A break in the investigation may have come from a taxi driver who took the suspects to the airport.
The driver contacted authorities after seeing surveillance footage and gave them the address where he picked the men up, according to two U.S. officials briefed on the investigation.
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.
Hours later, they were still combing through the building for evidence.
Security was high. At one point, a helicopter hovered overhead, carrying a sniper with a weapon trained on the building.
As masked, armed officers stood guard outside the building, the burst of camera flashes inside could be seen from the street below. Officers left the building carrying bags of evidence they loaded onto vehicles.
Ties to Paris attacks?
A Belgian government representative told CNN that 10 people were killed and 100 wounded at Brussels' international airport. At least 20 people died and 130 were wounded at the Maelbeek metro station, officials said.
The blasts sent wounded people fleeing into the streets, spurred evacuations of nuclear plants and transit hubs and led to raids in some areas as authorities searched for suspects and evidence.
Authorities in Belgium have been trying to crack down on terror threats for months as they raided homes in the area in search of suspects. Tuesday's violence came just days after investigators closed in on Europe's most wanted man, Paris attacks suspect Salah Abdeslam, who was hiding out in a Brussels suburb.
The "working assumption" is that the attackers came from the network behind November's massacres in Paris
, which left 130 dead, Belgian security sources said, while cautioning it is very early in the latest investigation. ISIS also claimed responsibility for those attacks.
As doctors treat the wounded and authorities search for suspects, a key question remains unanswered: Could the attacks have anything to do with Abdeslam's arrest
On Tuesday, Belgian Prime Minister Charles Michel deflected a question about whether there is any link between the attacks and the Belgium-born French citizen's capture, saying it is too early to tell.
Michel said Tuesday he had "no information" about who was responsible for the attack, adding that authorities will find that out, but now their focus is on caring for the victims.
Two senior U.S. officials told CNN they believe the Belgium attack is tied to the same network as Abdeslam.
One of the two airport explosions happened outside security checkpoints for ticketed passengers and near the airline check-in counters, an airline official briefed on the situation said.
The subway station blast happened about an hour later in the Brussels district of Maelbeek, near the European Quarter, where European Union institutions are based.
"We were fearing terrorist attacks," Michel told reporters Tuesday. "And that has now happened."
Belgium is no stranger to terror
. "The Belgians have been sitting on a ticking time bomb," a U.S. counterterrorism official said, given all those who have traveled from the small European nation to Syria and Iraq to join ISIS
, then possibly come back home.
But for survivors of Tuesday's blasts, the repeated warnings from officials in recent months didn't dull the shock of seeing the carnage.
"You cannot believe it; you cannot believe it," said Jef Versele, who was in the airport's departure hall when bombs exploded there. "It was so insane. Not in my backyard."
The second blast inside the airport blew out windows, created a lot of smoke and caused parts of the ceiling to fall, he added.
"People were on the floor," Versele said, estimating he saw 50 to 60 who were thrown to the ground and didn't seem to be able to walk.
Anthony Barrett saw the wounded carried out on stretchers and luggage carts as he watched from his hotel across from the terminal building.
"I could see people fleeing," he said.
'We remain united'
After the attacks in Brussels, the home of NATO and the capital of the European Union, leaders inside Belgium and beyond vowed not to back down in their fight against terror.
In Belgium, where officials declared three days of national mourning, Michel offered a resolute message to those who supported and cheered the attackers.
"To those who have chosen to be barbarous enemies of freedom, democracy and fundamental values ... we remain united as one," Michel said. "We are determined to defend our freedoms and to protect our liberties."
In its message claiming responsibility, ISIS noted that Belgium is one of the nations "participating in the international coalition against the Islamic State."
A Twitter post widely circulated by prominent ISIS backers Tuesday featured the words, "What will be coming is worse."