The MBTA is trying to figure out what caused a massive power outage that shut all four subway lines and left hundreds of people stranded in trains shortly before 9 a.m. today.

The T has managed to restore power but Boston Police say the Red Line was still out of commission as of 9:50 because of signal problems.

"Power is being restored, but all lines are experiencing delays," T spokesman Joe Pesaturo said around 9:30 a.m. "An investigation into the cause of the outage is underway." On its Web site, the T was advising people not to take the T but to find other ways to get around.