BRUSSELS — Authorities captured a suspect linked to the Brussels bloodshed Wednesday following a massive manhunt, Belgian media reported, as details emerged of the suicide attackers: two brothers who brought chaos and bloodshed to the city at the heart of European unity.

Belgian broadcaster RTBF said police arrested the suspect, identified in the report as 24-year-old Najim Laachraoui but later retracted the identification. They don’t know who was arrested.

European security officials had previously described Laachraoui as a suspected Islamic State bombmaker. His alleged role in Tuesday’s carnage in Brussels remains unclear.

Laachraoui, a Belgian who was born in Morocco and raised in the Schaerbeek neighborhood, is believed to have trained in Syria and then returned to Europe.

His DNA was found on one of the explosives belts from November’s Paris attacks, and he is thought to have traveled at one point with Salah Abdeslam, the only surviving suspect believed to have played a direct role in the Paris massacre. Abdeslam was captured last week.

Meanwhile, the brothers who waged the suicide blasts were identified by RTBF as Khalid and Brahim Bakraoui, who both had criminal records but had not been on watch lists as potential terror threats.

A senior Belgian official confirmed the men were responsible for two of Tuesday’s blasts: one detonating a nail-spewing bomb at an airport departures hall and the other on a subway train in attacks linked to the Islamic State that left at least 31 people dead.

The suspected third bomber has not been identified publicly, and the hunt also reached out in other directions.

Another man accompanied two of the bombers to the airport, along with luggage apparently heaving with explosives, but is believed to remain at-large. European authorities were also hunting for a suspected Belgian bombmaker who trained in Syria with the Islamic State and later slipped back into Europe.

Tuesday’s mass killings added Brussels to a somber list of European capitals that have been struck in the past year by deadly attacks either perpetrated or inspired by the Islamic State, including Paris and Copenhagen.

Authorities had been bracing for an attack in Belgium for months as the country has struggled to stem a tide of homegrown extremism and as the Islamic State has repeatedly threatened to hit Europe in its core.

But when the attacks finally came, the magnitude was stunning. The day’s violence represented the worst on Belgian soil since World War II.

More than a dozen people were killed, and several others were injured, after explosions at an airport and metro station in the Belgian capital.

“What we had feared has happened,” said Belgian Prime Minister Charles Michel. “This is a black moment for our country.”

The apparently coordinated explosions created a renewed sense of threat that spilled far beyond Brussels, as authorities boosted police patrols in cities such as Paris, London and Washington.

In a reflection of the heightened state of anxiety across Europe, the airport in the French city of Toulouse was briefly evacuated Wednesday morning for security reasons. It later reopened.

The targets in Brussels – homes of the European Union and NATO – to have been chosen for their symbolic value and for their ease of access.

The attackers first struck with twin bombings at the international airport, where early-morning travelers were preparing to board flights linking Brussels to cities across the continent and around the world. An hour later, a subway car transiting beneath the modernist glass-and-steel high-rises that house the E.U. burst with smoke and flame.

In addition to the dead, about 250 people were injured, Belgian officials said.

Some of the injured lost limbs as shrapnel from the blasts radiated through packed crowds. Cellphone video recorded in the minutes after the airport blasts showed children cowering on a bloody floor amid the maimed and the dead.

Images from a subway station revealed desperate scenes as people dressed for a day’s work stumbled from the mangled wreckage into a smoke-drenched tunnel.

Authorities acknowledged that they had been readying for an attack. But nothing like this, they said.

“We never could have imagined something of this scale,” Interior Minister Jan Jambon told Belgian television station RTL.

And even as the country tried to recover from the trauma of Tuesday’s strikes, there was evidence that more could be on the way.

The man being sought by police accompanied two of the bombers to the airport, according to a senior Belgian official who spoke on the condition of anonymity to discuss sensitive details of the case. The taxi driver who transported them said they were hauling particularly heavy luggage that investigators believe was packed with explosives.

The taxi driver who drove the men to the airport later led police to an apartment in the Schaerbeek area of Brussels, where investigators found explosive devices loaded with nails and chemicals, along with an Islamic State flag.

“It was exactly the same type of bomb as at the airport,” the senior official said.

Belgian police released surveillance images of three men pushing luggage carts at Brussels Airport. The prosecutor’s office said two of them – dressed in black with black gloves on their left hands, probably to conceal detonators – had blown themselves up. But the third, dressed in white, was on the loose. His identity was unknown, and despite a nationwide hunt – with heavily armed officers combing the streets and checkpoints at Belgian borders snarling traffic for miles – the suspect remained at large Tuesday night.

Within hours of Tuesday’s assault, the Islamic State asserted responsibility for the attacks, according to a statement posted on the Amaq Agency, a website believed to be close to the extremist group. The message said Belgium was targeted because of its participation in an international coalition battling the group in Syria and Iraq. U.S. and European security officials said they believed the claim to be credible.

In Washington, State Department spokesman John Kirby said U.S. citizens were among the injured, but he would not say how many. No Americans are known to have died in the attacks, although that information may change, he said.

The State Department also issued an alert on traveling in Europe, urging Americans to avoid crowded places and to exercise caution during religious holidays and at large festivals or events.

Europe has struggled mightily with spillover from the churning conflict in Syria. Thousands of European citizens have traveled there to fight in a war that has become a focal point for jihadists around the world. Many have returned to Europe radicalized. Europe has vowed to confront them.

“This is a kind of scenario every capital in Europe feared since the November attacks last year. A mixture of foreign fighters coming back with experience, local sympathizers on the other hand,” said Rik Coolsaet, a terrorism expert at Ghent University who has advised the Belgian government on how to fight radicalization. “You have such a large number of soft targets, and you cannot secure all of them.”

Birnbaum reported from Moscow. James McAuley and Anthony Faiola in Brussels, Daniela Deane and Karla Adam in London, and Brian Murphy, Carol Morello and Matt Zapotosky in Washington contributed to this report.