Federal investigators investigating the deadly midair collision near Ronald Reagan Washington National Airport described a strained, high-stress air traffic culture where controllers relied on experience,

The National Transportation Safety Board is probing air traffic control staffing, flight volume, training and other procedures in the investigation into the collision between an Army helicopter and an American Airlines regional jet that killed 67 people.

The F.A.A. has since been training on-call specialists to be snappier about administering alcohol and drug tests after a serious incident, an official said.