HYATTSVILLE, Md. (AP) - Metro officials say they still don't know what caused a software program tracking trains to shut down twice over the weekend, and it's possible it could happen again.

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All trains on the Washington-area transit network were halted for a half-hour each time the system failed.

Dave Kubicek, Metro's deputy general manager for operations, says the computer system vendor and Metro engineers and maintenance staffers are working around the clock to figure out what caused the problem. He says he can't be sure it won't happen again.

Kubicek says officials do not believe the failures were caused by a cyber-attack.

The glitch did not affect Metro's signaling system, which ensures that trains maintain enough space between them. But it affected Metro's ability to see where trains were on the lines.