Explosions that tore through the departures area at Brussels' international airport and a subway station in the heart of the Belgian capital on Tuesday killed at least 30 people, injured 230 and set the European Union capital further on edge.
Islamic State extremists claimed responsibility for the attacks, which officials believe were carried out by suicide bombers less than a week after the arrest in Brussels of a key suspect in the Paris terrorist attacks.
In the hunt for the perpetrators, police found an explosive device containing nails, chemical substances and an Islamic State flag during a raid in the Brussels neighbourhood of Schaerbeek, federal prosecutors said Tuesday evening, without providing further details.
Media reported that a taxi driver pointed investigators to the alleged terrorists' hiding place.
The driver had picked up the suspects at an apartment in Schaerbeek and drove them to the airport. He noticed that the men did not want him to help them with their luggage, broadcaster VRT reported.
Federal prosecutor Frederic Van Leeuw said it was too early to establish a link between the attacks in Brussels and the November attacks in Paris, which were also claimed by the Islamic State.
The Sunni jihadist organization, which controls swathes of territory in Iraq and Syria, said a group of its "soldiers" had attacked "carefully selected targets" in "crusader Belgium, which has not stopped attacking Islam and its people."
Belgium is part of the US-led coalition that has carried out airstrikes against Islamic State.
The group threatened further attacks on members of the coalition in a statement posted on social media by supporters and described by the US-based monitoring group SITE Intel as a "formal communique." It said that "what is coming will be harsher and bitterer."
Authorities questioned how the intelligence gathering apparatus could have missed any clues about the bombing plot, especially in light of the arrest of Paris bombing suspect Salah Abdeslam, which already had put Belgium on high alert.
In Germany, Interior Minister Thomas de Maiziere said European countries must get better at connecting and sharing data collected by their various national immigration and security authorities.
"The most important tool for prevention is information. We have to share the information that we have," he said on German public channel ZDF.
Top Belgian officials said the attacks had only stealed their resolve to protect the country's way of life.
"I want to tell ... those who have chosen to be barbaric enemies of liberties, of democracy, of fundamental values that we will stay united, that we will be fully mobilized ... with a full determination to act to protect our liberties, our ways of living," Prime Minister Charles Michel said in Brussels.
Belgian prosecutors believe two men carried out a suicide bombing at the airport, while a third suspect caught on video surveillance is being sought.
In the first attack two consecutive explosions occurred in the airport's departure hall around 8 am (0700 GMT) with about 1,000 people present. It left at least 10 people dead and some 100 people injured, according to Belgium's crisis centre.
The explosion at the Maelbeek metro station went off just over an hour later in a subway train stopped in the station, leaving at least 20 people dead and some 130 injured, the crisis centre said.
The US State Department said it could not confirm Islamic State's claim of responsibility but noted that the group is "capable of this sort of depravity."
People laid down flowers in the centre of the Belgian capital Tuesday as the northern European country began three days of national mourning.
A group of people gathered in Place de la Bourse held a banner with the French and Flemish slogans "Je suis Bruxelles - Ik ben Brussel" (I am Brussels).
Police spent much of the day searching for weapons and suspects in the sprawling airport building, located about 15 kilometres from the Brussels centre.
There were three bombs used during the airport attack, but one did not explode, the governor of the Flemish Brabant province, Lodewijk De Witte, was quoted by the Belga news agency as saying. The bombs contained nails, an official from a hospital treating some of the victims told Belga.
The crisis centre said there would be no incoming or outgoing flights until at least noon on Wednesday.
Public transport in Brussels was also shut down for most of Tuesday, although it came back on line by the evening. Train traffic to and from Brussels also was resuming, and schools were expected to be open on Wednesday.
Military reinforcements have been deployed and controls are being carried out at Belgium's borders.
The country raised its threat alert level to 4, indicating a "serious and imminent" threat. EU institutions and the NATO military alliance, which is headquartered in Brussels, also increased their threat levels.