Belgian prosecutors launched a public appeal Thursday seeking any information on "the man in a hat" seen before the Brussels Airport suicide bombings that killed 16 people.
Two officials say the suspected bombmaker in the Paris attacks was one of two suicide bombers who died in the Brussels airport blasts.
It was a show of solidarity from thousands of miles away on Wednesday.
The Brussels attacks have led to a higher security presence in the United States, especially at all major transit hubs which includes the Philadelphia International Airport.
Explosions rocked the Brussels airport and the subway system Tuesday, just days after the main suspect in the November Paris attacks was arrested in the city, police said.
Islamic extremists struck Tuesday in the heart of Europe, killing at least 34 people and wounding scores of others in back-to-back bombings of the Brussels airport and subway that again laid bare the continent's vulnerability to suicide squads.