SAN FRANCISCO (AP) — City and utility officials vowed Friday to prevent a recurrence of two blackouts that struck Candlestick Park during a nationally televised Monday night football game.

Pacific Gas & Electric Co. said the initial blackout that delayed the start of the game by about 20 minutes occurred when a wire that carries the stadium's main power supply broke, creating a display of sparks shown during the ESPN broadcast.

San Francisco Mayor Ed Lee said the second blackout that stopped play for about 15 minutes in the second quarter was caused by a malfunctioning switch involving the backup power supply.

In a letter to the 49ers, PG&E President Anthony Earley said the company was fixing the failed line and installing monitoring equipment, and will perform a test to ensure the lights stay on if it is necessary to switch to backup power.

Technicians also are performing tests on the splice sleeve that failed and caused the line to break in two, Earley said.

In a separate letter to the 49ers, the San Francisco mayor said crews were replacing the switch that caused the second blackout and would perform tests to ensure it functions properly.

The power failure also brought renewed attention to the 49ers' intention to move the team out of 51-year-old Candlestick to a new stadium outside city limits in nearby Santa Clara, which has secured most of the financing needed to build a new venue.