Police boats combed the banks of the Potomac River on Friday, slowly scanning the shoreline under rainy skies as investigators sought clues into the midair collision that killed 67 people and raised questions about air traffic safety around the nation’s capital. No one survived the Wednesday night collision between the commercial airliner and an Army helicopter. The remains of 41 people had been pulled from the river as of Friday afternoon, including 28 that had been positively identified, Washington, D.C., Fire Chief John Donnelly Sr. said at a news conference. He said next of kin notifications had been made to 18 families, and that he expects the remains of all 67 people who died to eventually be recovered.