Islamic State claimed responsibility for suicide bomb attacks on Brussels airport and a rush-hour metro train in the Belgian capital on Tuesday which killed at least 30 people, with police hunting a suspect who fled the air terminal.
Police issued a wanted notice for a young man in a hat who was caught on CCTV pushing a laden luggage trolley at Zaventem airport alongside two others who, investigators said, had later blown themselves up in the terminal, killing at least 10 people.
Officials said 20 died on the metro train close to European Union institutions. It was unclear still what caused the blast but a news agency linked to Islamic State said that too was a suicide attack.
The coordinated assault triggered security alerts across Europe and drew global expressions of support, four days after Brussels police had captured the prime surviving suspect in Islamic State's attacks on Paris last November.
Belgian authorities were still checking whether the attacks were linked to the arrest of Salah Abdeslam, according to Federal Prosecutor Frederic Van Leeuw, although U.S. officials said the level of organisation involved suggested they had previously been in preparation.
Explosives and an Islamic flag were found after a flat was raided a week ago where a fresh fingerprint of Abdeslam's had put police on his trail. It was not clear if Abdeslam had been involved at that stage in the airport attack plan.
A bomb and an Islamic State flag were also found later on Tuesday in a flat in Brussels.
"A photograph of three male suspects was taken at Zaventem. Two of them seem to have committed suicide attacks. The third, wearing a light-coloured jacket and a hat, is actively being sought," Van Leeuw told a news conference.
A government official said the third suspect had been seen running away from the airport building. Local media said police had found an undetonated suicide vest in the area.
Police issued a wanted notice on Monday, after the questioning of Abdeslam, identifying 25-year-old Najim Laachraoui as linked to the Paris attacks. The poor quality of the images left open whether he might be the person caught on the airport cameras.
A witness said he heard shouts in Arabic and shots shortly before two blasts struck in a packed airport departure lounge at the airport. Belgian media published the security camera picture of three young men pushing laden luggage trolleys. Police later issued the same photograph, showing only one of the three.
"If you recognise this individual or if you have information on this attack, please contact the investigators," the notice read. "Discretion assured."
Police operations were under way at several points in the city but a lockdown imposed immediately after the attacks was eased and commuters and students headed home as public transport partially reopened.
Islamic State issued a statement claiming responsibility: "We promise the crusader alliance against the Islamic State that they will have black days in return for their aggression against the Islamic State," the jihadist group said.
Belgium, home to the European Union and the headquarters of the NATO military alliance, has sent warplanes to take part in operations against Islamic State in the Middle East.
Austrian Horst Pilger, who was awaiting a flight with his family when the attackers struck, said his children had thought fireworks were going off, but he instantly knew an assault was underway.
"My wife and I both thought 'bomb'. We looked into each other's eyes," he told Reuters. "Five or 10 seconds later there was a major, major, major blast in close vicinity. It was massive."
Pilger, who works at the European Commission, said the whole ceiling collapsed and smoke flooded the building.
Security services found and destroyed a third bomb after two blasts at the airport killed at least 10 people and injured around 100, the provincial governor of Brabant Flanders said. Belgian media gave death tolls as high as 14 at the airport.
The metro station blast killed a further 20 people and injured roughly 130, according to a provisional toll from the national crisis response centre.
U.S. President Barack Obama led calls of support to Belgian Prime Minister Charles Michel after Brussels had gone into a state of virtual lock-down. "We must be together regardless of nationality or race or faith in fighting against the scourge of terrorism," Obama told a news conference in Cuba. "We can and we will defeat those who threaten the safety and security of people all around the world."
Michel spoke at a Brussels news conference of a "black moment" for his country. "What we had feared has come to pass."
The blasts occurred after the arrest in Brussels of a suspected participant the Paris attacks that killed 130 people. Belgian police and combat troops on the streets had been on alert for reprisal but the attacks took place in crowded areas where people and bags are not searched.
All public transport in Brussels was initially shut down, as it was in London during 2005 Islamist militant attacks there that killed 52.
Authorities appealed to citizens not to use overloaded telephone networks, extra troops were sent into the city and the Belgian Crisis Centre, clearly wary of a further incident, appealed to the population: "Stay where you are". Brussels airport will remain closed on Wednesday, its chief executive Arnaud Feist told reporters.
Public broadcaster VRT said police had found a Kalashnikov assault rifle next to the body of an attacker at the airport. Such weapons have become a trademark of Islamic State-inspired attacks in Europe, notably in Belgium and France, including on Nov. 13 in Paris.
Alphonse Youla, 40, who works at the airport, told Reuters he heard a man shouting out in Arabic before the first explosion. "Then the glass ceiling of the airport collapsed." "I helped carry out five people dead, their legs destroyed," he said, his hands covered in blood.
Others said they also heard shooting before the blasts. A witness said the blasts occurred at a check-in desk. Video showed devastation in the hall with ceiling tiles and glass scattered across the floor. Bloodied bodies lay around.
Some passengers emerged from the terminal with blood spattered over their clothes. Smoke rose from the building through shattered windows and passengers fled down a slipway, some still hauling their bags.
Britain, Germany, France and the Netherlands, all wary of spillover from conflict in Syria, were among states announcing extra security measures. Security was tightened at the Dutch border with Belgium.
The blast hit the train as it left Maelbeek station, close to EU institutions, heading to the city centre. VRT carried a photograph of a metro carriage at a platform with doors and windows completely blown out, its structure deformed and interior mangled and charred.
A local journalist tweeted a photograph of a person lying covered in blood among smoke outside the station. Ambulances were ferrying the wounded away and sirens rang out across the area.
"We are at war and we have been subjected to acts of war in Europe for the last few months," French Prime Minister Manuel Valls said.
Train services on the cross-channel tunnel from London to Brussels were suspended. Britain advised its citizens to avoid all but essential travel to Brussels.
Security services have been on a high state of alert across western Europe for fear of militant attacks backed by Islamic State, which claimed responsibility for the Paris attack.
While most European airports are known for stringent screening procedures of passengers and their baggage, that typically takes place only once passengers have checked in and are heading to the departure gates.
Abdeslam, the prime surviving suspect for the Paris attacks on a stadium, cafes and a concert hall, was captured by Belgian police after a shootout on Friday. Interior Minister, Jan Jambon, said on Monday the country was on high alert for a revenge attack.