Federal investigators probing January’s fatal air collision near Ronald Reagan Washington National Airport have called for restrictions on military helicopters to make the area safer for commercial passengers.

Federal investigators looking for the cause of the January collision between a passenger jet and an Army helicopter near Washington, D.C., that killed 67 people recommended a ban on some helicopter flights Tuesday to improve safety.

The National Transportation Safety Board said the separation between helicopters and jets in the area was insufficient and increased “chances of a midair collision.”