Belgian police carried out raids this evening in search of suspects involved in attacks on Brussel's airport and a subway station this morning that killed at least 34 people and injured another 181.
According to the Belgian federal prosecutor's office, officers found an explosive device containing nails, chemicals and an Islamic State flag while carrying out searches in the Schaerbeek area.
Both attacks have since been claimed by the Islamic State (IS) group.
The prosecutor said that two attackers blew up at Brussels airport, and a third is being hunted.
Belgian police have now issued a wanted notice for the third attacker, and issued a photograph of a man, dressed in white and wearing a hat, as he pushed a luggage trolley.
The attacks at Brussel's airport occurred around 8am local time, and killed at least 14 people and injured another 81. Shortly afterwards, another blast struck a metro station in the Belgian capital, close to European Union (EU) institutions, which left an estimated 20 dead and more than 100 people wounded.
The attack at the airport, located in Zaventem, 11 kilometers (6.8 miles) northeast of the city center, involved two explosions that rocked the departure hall and another just outside the airport's Starbucks. A third bomb was later found on the airport premises. Security services destroyed the undetonated device, according to the provincial governor of the Brabant Flanders region.
Social media showed pictures of smoke rising from the airport departure hall where windows had been shattered by the blasts. Passengers were seen running away down a slipway.
The second attack took place at Maelbeek metro station in southeastern Brussels as a train was leaving the station traveling towards the center of the European Quarter. Victims reportedly attempted to climb out of the train windows.
Islamic State claimed responsibility for the attacks through a statement on its Amaq Agency, which is linked to the group. The group identified Belgium as "a country participating in the international coalition against the Islamic State." According to the statement "several" fighters were involved in the airport attack, while it said the metro station bombing was carried out by one suicide bomber.
Belgium and France had already been on alert for any reprisal action after the Friday arrest of Salah Abdeslam, the prime surviving suspect for November's Paris attacks on a stadium, cafes, and a concert hall. Abdeslam was captured by Belgian officers after a shootout on Friday and France deployed 1,600 police officers and gendarmes at the border and at major transit hubs following the arrest.
Belgium's Interior Minister, Jan Jambon, said on Monday the country was on high alert for a possible revenge attack following the capture of 26-year-old Abdeslam.
"We know that stopping one cell can... push others into action. We are aware of it in this case," he told public radio.
Belgium's Prime Minister Charles Michel addressed the attacks this morning, calling them "blind, violent, and cowardly."
"We were fearing terrorist attacks, and that has now happened," he said. "We must face this challenge in solidarity, united, together."
Speaking from the Élysée presidential palace, French Interior Minister Bernard Cazeneuve announced that an extra 400 police officers and gendarmes had been deployed at Roissy-Charles-de-Gaulle and Orly airports, as well as in subway and train stations, and other sensitive sites in and around the French capital.
"France and Belgium are linked in horror," said French president François Hollande, adding that, "The war against terrorism must be fought across the whole of Europe."
"We must make sure we are united, on a European level, on the global level, and the most vital unity, on a national level," said Hollande. "We are all affected."
In Paris this evening, the Eiffel tower was lit up in the colors of the Belgian flag in solidarity with the victims of the attack.
In the US, President Barack Obama called for unity in the fight against terrorism, and Homeland Security Secretary Jeh C. Johnson said the Transportation Safety Administration was "deploying additional security to major city airports in the United States, and at various rail and transit stations around the country."