At about 11:00 eastern time, a Stinson 108-3 lost engine power after takeoff from Potomac Airfield and made a forced landing into trees. The pilot sustained minor injuries and the passenger was seriously injured. The pilot said he had just purchased fuel and was returning to his home airport about two miles to the east. The engine ran rough immediately on startup, but then smoothed out. He departed runway 24 but the engine lost all power on initial climb. The pilot believed he had a magneto problem.