SOMERVILLE, Mass. — A fire on a Boston-area public transit train that prompted one passenger to jump into a river and others to scramble out of windows appears to have been caused by a metal panel on the train’s base that came loose and touched the electrified third rail, the system’s general manager said.

No one was hurt Thursday morning when the Massachusetts Bay Transportation Authority’s inbound Orange Line train caught fire while on a bridge crossing the Mystic River just north of Boston on approach to the Assembly station in Somerville.

MBTA General Manager Steve Poftak said at a news conference late Thursday that the preliminary indication is that a 1-by-6-foot panel that he called a “sill” that runs along the base of the train came loose, touched the high-voltage rail and caused sparks that ignited other materials.

It appears passengers removed four windows during the escape, and most of the roughly 200 passengers on board were walked to safety by MBTA personnel, Poftak said. The woman who jumped into the river declined medical attention.

The power to the third rail was turned off in less than two minutes, he said.

The 43-year-old train car in question had been inspected less than a month ago, which included an inspection of the panel that came loose, he said. After the fire, the same panel on every other in-service Orange Line car was inspected, and no issues were found, Poftak said.

The investigation is ongoing, he said.

The fire is the latest in a string of dangerous problems with the troubled system.

The Federal Transit Administration launched a review of the subway system in April following several accidents in the past year that led to injuries or death. The federal agency last month issued a series of directives to immediately address “longstanding issues” with the system’s “overall safety program and safety culture.”