The National Transportation Safety Board is now investigating the aborted landings of two commercial flights approaching Reagan National Airport on Thursday as a U.S. Army Black Hawk helicopter was approaching the Pentagon nearby.

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.

U.S. Transportation Secretary Sean Duffy says it is “unacceptable” that two commercial aircraft had to abort landings at a Washington airport this week because of an Army Black Hawk helicopter that was flying to the Pentagon.