A decades-old cable that fell inside a subway tunnel Tuesday caused the halted Blue Line train that stranded hundreds of passengers under Boston Harbor, officials said Wednesday.

The head of the MBTA said more problems were uncovered during an investigation into a Blue Line train evacuation that happened on Tuesday in Boston and he vowed that the agency will work to fix them.Passengers were forced to walk through a dark tunnel in sweltering heat and humidity after the Blue Line train broke down.

An MBTA spokesperson said the train operator stopped the train after they spotted a downed wire in the tunnel.