Solidarity messages are written in chalk outside the stock exchange in Brussels on Tuesday, March 22, 2016. (Geert Vanden Wijngaert/AP)
A police officer directs passengers in a smoke filled terminal at Brussels Airport, in Brussels after explosions Tuesday, March 22, 2016. (Ralph Usbeck via AP)
The blown out windows of Zaventem airport are seen after a deadly attack in Brussels, Belgium, Tuesday, March 22, 2016. (Peter Dejong/AP)
In this photo provided by David Crunelle, an injured woman leaves the scene at Brussels airport, after explosions rocked the facility in Brussels, Belgium, Tuesday March 22, 2016. (David Crunelle via AP)
A woman is evacuated in an ambulance by emergency services after a explosion in a main metro station in Brussels on Tuesday, March 22, 2016. (Virginia Mayo/AP)
President Obama receives an update on the terror attacks in Brussels, Belgium. The President made the call from Havana, Cuba, March 22, 2016. (Official White House Photo by Pete Souza)
In this photo provided by Evan Lamos, passengers clamber from a metro carriage after explosions in Brussels Tuesday, March 22, 2016. (Evan Lamos via AP)
People walk away from the broken windows at Zaventem Airport in Brussels after an explosion on Tuesday, March 22, 2016. (Geert Vanden Wijngaert/AP)
A woman places candles in the shape of a heart outside the stock exchange in Brussels on Tuesday, March 22, 2016. (Geert Vanden Wijngaert/AP)
Two women wounded in Brussels Airport in Brussels, Belgium, after explosions were heard Tuesday, March 22, 2016. (Ketevan Kardava/Georgian Public Broadcaster via AP)
A man is wounded in Brussels Airport in Brussels, Belgium, after explosions were heard Tuesday, March 22, 2016. (Ketevan Kardava/Georgian Public Broadcaster via AP)
This photo shows the scene in Brussels Airport in Brussels, Belgium, after explosions were heard Tuesday, March 22, 2016. (Ketevan Kardava/Georgian Public Broadcaster via AP)
''People wounded outside #maelbeek #metro #brussels'' (dirkalaerts/Twitter)
''Metro Maelbee, fumee apres explosion.'' (''Maelbeek metro, smoke after explosion.'') (valerylerouge/Twitter)
The scene in Brussels, Belgium following deadly explosions. (dirkalaerts/Twitter)
A young girl looks out of the window of a bus after being evacuated from the Brussels airport. (Geert Vanden Wijngaert/AP Photo)
A police officer stands guard as people are evacuated from the Brussels airport. (Geert Vanden Wijngaert/AP Photo)
People stand near the Brussels airport after being evacuated following explosions that rocked the facility in Brussels, Belgium, Tuesday. (Geert Vanden Wijngaert/AP Photo)
People comfort each other after being evacuated from the Brussels airport. (Geert Vanden Wijngaert/AP Photo)
A police officer, right, stands guard as people are evacuated near the Brussels airport. (Geert Vanden Wijngaert/AP Photo)
People stand near the Brussels airport after being evacuated. (Geert Vanden Wijngaert/AP Photo)
People react as they walk away from Brussels airport after explosions rocked the facility in Brussels, Belgium, Tuesday, March 22, 2016. (AP Photo/Geert Vanden Wijngaert)
People walk away from Brussels airport after explosions rocked the facility in Brussels, Belgium, Tuesday, March 22, 2016. (AP Photo/Geert Vanden Wijngaert)
People walk away from Brussels airport after explosions rocked the facility in Brussels, Belgium, Tuesday, March 22, 2016. (AP Photo/Geert Vanden Wijngaert)
People react as they walk away from Brussels airport after explosions rocked the facility in Brussels, Belgium, Tuesday, March 22, 2016. (AP Photo/Geert Vanden Wijngaert)
People walk away from Brussels airport after explosions rocked the facility in Brussels, Belgium, Tuesday, March 22, 2016. (AP Photo/Geert Vanden Wijngaert)
People react as they walk away from Brussels airport after explosions rocked the facility in Brussels, Belgium, Tuesday, March 22, 2016. (AP Photo/Geert Vanden Wijngaert)
People walk away from Brussels airport after explosions rocked the facility in Brussels, Belgium, Tuesday, March 22, 2016. (AP Photo/Geert Vanden Wijngaert)
Smoke is seen at Brussels airport in Brussels, Belgium, after explosions were heard Tuesday, March 22, 2016. (Daniela Schwarzer via AP)