WASHINGTON --

Transit trains around the nation's capital could continue running manually for a year or more while Metro makes sure its automatic control system works properly following a deadly crash, the agency's general manager said Tuesday.

Nine people were killed and more than 70 injured June 22 when a moving train slammed into another train stopped on the tracks near the Maryland state line. Metro's train control system is supposed to prevent crashes and investigators are trying to determine what went wrong.

The National Transportation Safety Board has said Metro's signaling system failed to detect a test train stopped in the same place as one struck during the crash.

General Manager John Catoe said Tuesday it was unclear how long trains will operate manually, but when pressed he replied: "It could a month, a year or two years."

Catoe said crews have inspected 65 percent of the Metro's 3,000 track signals to make sure there aren't similar problems elsewhere. He said so far, none has been found.

As an added precaution, he said a group of independent experts was being brought in to examine the rail system's signals and other equipment.

Metro also is reconfiguring its trains by putting older rail cars in the middle. The striking train, which sustained most of the damage, was made up of these 1,000-series cars dating to the 1970s.