The Jan. 29 collision over the Potomac River in Washington, DC — which preliminary reports have blamed on the military Black Hawk flying too high — was the deadliest US air disaster since 2001, and caused officials to change the flight path of helicopters near the airport.

Transportation Secretary Sean Duffy says he will ask Defense Department officials why an Army Black Hawk helicopter violated flight restrictions at Ronald Reagan Washington National Airport.

A Virginia-based helicopter unit will pause flights pending the outcome of an internal inquiry, an Army spokeswoman said.