Belgian police launched a massive manhunt for a suspected Brussels attacker whose image was captured in a CCTV camera, after the twin bombings at Zaventem airport and another blast at a metro train killed about 35 people on Tuesday.
The Islamic State claimed the responsibility for Brussels bombing and warned "crusader states" allied against it were in for more such attacks.
Isis said in statement 'caliphate soldiers' targeted the airport and metro station "strapped with suicide vests and carrying explosive devices and machineguns."
"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 statement said.
The airport explosions happened in quick succession shortly after 08:00 local time. An hour after the twin blasts at the airport, a bomb went off in a rush hour metro train near the European Union headquarters in Maelbeek, killing at least 20 people.
Isis claimed it was behind the metro station attack as well, saying that too was a suicide bombing.
Belgian police said about 250 people have been injured in the three blasts.
"This is a day of tragedy, a black day .. I would like to call on everyone to show calmness and solidarity," Belgian Prime Minister Charles Michel said.
Raid in Schaerbeek
A massive search was underway at night in the northern borough of Schaerbeek after a taxi driver reported to police that he had driven three people from that area to the airport Tuesday morning. He said he grew suspicious of them as they did not allow him to tough their baggage, Reuters reported, citing local TV channel VTM.
Security footage at the Zaventem airport showed two young men in dark clothes and wearing gloves on one hand walking alongside a man in hat -- all of them pushing laden trolleys.
Investigators said the men in black clothes later blew themselves up in the airport's departure terminal, killing more than 10 people. The third one apparently escaped the scene.
"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," Belgian Federal Prosecutor Frederic Van Leeuw said, Reuters reported.
Reports said the third man, the one in a light colored jacket, was seen running away from the airport building, apparently leaving behind an explosive device which the police found and neutralized.
Police found a nail bomb and an Islamic State flag from an apartment in Schaerbeek, from where the suspected bombers came.
Explosives and Islamic flags were found during a raid in southern Brussels last week to flush out terrorists involved in the Paris attacks in November that killed 130 people.
Belgian police later arrested Salah Abdeslam, who is directly involved in Paris attacks. Prosecutors who questioned Abdeslam again after the Brussels attacks on Tuesday, haven't said conclusively if he was involved in the planning of the attacks.