WATERVILLE (AP) – Police are investigating an assault at Colby College in which an 18-year-old was attacked by a man thought to be wearing a ski mask and a hooded sweatshirt.

The attack happened shortly before 9 p.m. Friday while the woman was walking behind a dormitory in the middle of campus, Waterville Police Chief Joe Massey said.

The assailant grabbed the woman by the hair and pulled her to the ground, but she screamed and drew attention, Massey said. When another student who heard the commotion hollered out, the attacker fled, the chief said.

The student was shaken up, but did not require immediate medical attention.

Waterville police, along with Colby College security and Maine State Police, searched the campus with a tracking dog and all-terrain vehicles, but didn’t turn up a suspect.

The assailant was described as being about 5 feet 9 inches tall, with a slender to medium build.

But a full description was hard to come by because the man’s identity was obscured and it was dark out, Massey said.

Colby has put out advisories urging students to use the school’s 24-hour escort services and advising them to walk with other people when moving around campus. Waterville police have stepped up patrols.

“We’re not in a high-risk area, but when something like this happens, you have to step it up,” said Colby spokesman Steve Heacock.

The assault is believed to be an isolated incident, Massey said, but he advised students to avoid areas at night that are not well-lit and to stay clear of buildings that are not in use.

“Students need to take reasonable precautions,” he said.

Four years ago, a 21-year-old Colby student from Medway was abducted from campus and found dead a day later in the neighboring town of Oakland.

Edward Hackett, a parolee from Utah, later pleaded guilty in the kidnapping and murder of Dawn Rossignol. He is serving a life term in prison.

At the time of the killing, Hackett was on parole for kidnapping and robbery in Utah and was living with his parents in Vassalboro.

AP-ES-10-15-07 1340EDT