Police officers and investigators stand outside the Maelbeek - Maalbeek subway station in Brussels on March 22, 2016. (AFP Photo)
The FBI and New York police are sending detectives to investigate the Brussels attacks because US citizens are among the casualties, a senior police officer announced on Tuesday.
John Miller, New York deputy police commissioner for intelligence and counterterrorism, made the remarks as the US government, state and city authorities stepped up security at major US airports and transit hubs.
Three American missionaries were seriously wounded while accompanying a French colleague to Zaventem airport on her way to Ohio, the Mormon church announced earlier on Tuesday.
"Because there are Americans among the casualties and US persons in the attack in Belgium, there will be an FBI investigation coming out of the New York City Joint Terrorism Task Force," Miller told reporters.
"We expect agents and New York City detectives, or Task Force officers, to be gearing up to leave for that investigation overseas as early as tonight or tomorrow."
New York police detectives and FBI agents from the task force have been involved in numerous international terrorism investigations, including the US embassy bombings in East Africa in 1998 and the attack on the USS Cole in 2000.
Belgium launched a huge manhunt after bombings claimed by the ISIS group ripped through Brussels airport and a metro train, killing around 35 people.