Officials say the conditions of the Potomac River are complicating recovery efforts of the bodies of the 67 presumed dead in a mid-air collision between American Airlines flight 5342 from Wichita and a military Black Hawk helicopter.

The flight data recorder and cockpit voice recorder, known as the black boxes, may offer clues about what went wrong before the crash.

Multiple 911 callers reported the crash near the river just before 8:55 p.m., according to the Metropolitan Police Department and the District of Columbia Fire and EMS.