WASHINGTON (AP) — FBI Director Jim Comey says the agency used its aircraft above Ferguson, Missouri, last year to help local law enforcement keep track of unrest on the ground.

Comey did not go into details during a House Homeland Security Committee hearing on Thursday, including how long the surveillance lasted.

But in response to questioning, he did say that the FBI uses airplanes during investigations of specific suspects in criminal, terrorism and espionage investigations and to help local police during emergencies.

He said the FBI never uses its planes for mass surveillance. When planes are flown above large crowds and gatherings at the request of local law enforcement, the purpose is to look for violence and trouble spots. A plane was also flown above riots in Baltimore last April.