WASHINGTON (AP) — The National Transportation Safety Board says the placement of a helicopter route so close to Reagan National Airport’s secondary runway was a key factor in causing a midair aircraft collision that killed 67 people near Washington, D.C., a year ago.

The NTSB said Tuesday that air traffic controllers’ over-reliance on asking pilots to avoid other aircraft also contributed to an American Airlines jet and an Army Black Hawk helicopter colliding a year ago. The Federal Aviation Administration has taken steps to ensure helicopters and planes no longer share the same crowded airspace around the nation’s capital.

An NTSB investigator says air traffic control had a big workload around the time of the crash.