At least two locations were hit: There were two explosions at Brussels’s Zaventem airport at about 8 a.m. local time (3 a.m. ET) and at least one more at Maelbeek metro station, which is close to the main European Union buildings.Thirty-four people were killed in attacks on Brussels airport and a rush-hour metro train in the Belgian capital on Tuesday, according to public broadcaster VRT, triggering security alerts across Europe and bringing some cross-border traffic to a halt.

A witness said he heard shouts in Arabic and shots shortly before two blasts struck a packed airport departure lounge at Brussels airport. The federal prosecutor said one of the explosions was probably triggered by a suicide bomber.

The blasts occurred four days after the arrest in Brussels of a suspected participant in November militant attacks in Paris that killed 130 people. Belgian police and combat troops on the streets had been on alert for reprisal but the attacks took place in crowded areas where people and bags are not searched.

Public transportation has been shut and peoplehave been asked to stay in place.Belgium has raised its terror alert system from three to four, its highest level.Security has been stepped up across Europe in the wake of the attacks.