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 organization 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-colored 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 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 recognize 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 center.