BOSTON -- An 18-year-old high school student was struck and killed by a commuter rail train in Belmont.

The incident happened at about 12:40 p.m. Thursday on train tracks behind Belmont High School.

The female student, who attends Belmont High School, was struck by an inbound Fitchburg Line Commuter Rail train.

MBTA spokesman Joe Pesaturo said the incident happened just east of the Belmont Station on a tree-lined section of track, about 250 yards from the closest designated area for crossing the tracks.

Investigators said that the girl walked out onto the tracks during class.

"We don’t know what the cause is," MBTA Police Deputy Chief Joe O'Connor said. "We are going to do a slow, thorough investigation."

A fence borders the tracks and stands feet from the school. Some Belmont students said that they do cut through the fence to cross the tracks.

"We do the best we can to make sure it doesn't happen. Sometimes (there's) a tragic accident, and this is what it is," Belmont Police Department Chief Richard McLaughlin said.

The victim's name has not been released.