The U.S. Federal Aviation Administration is indefinitely restricting helicopter flights near Washington Reagan National Airport after a deadly collision between an American Airlines regional jet and Army Black Hawk killed 67 people.

The National Transportation Safety Board said it was too soon to speculate the cause of the deadly crash. Skaters, parents, and coaches from the Skating Club of Boston were aboard the flight.

A preliminary safety report from the Federal Aviation Administration reportedly found that air traffic control staffing was abnormally low at Ronald Reagan Washington National Airport on the evening of Wednesday, Jan. 29, according to The New York Times, which obtained a copy of the report.