A female undergraduate was sexually assaulted by an unknown attacker in a Yard dorm early Thursday morning, according to the Harvard University Police Department (HUPD).

The victim described her attacker as a six-foot tall white male with crew-cut hair. No further description was available, as the attack occurred in a student's darkened dorm room, Riley said.

He said police officers believe the dorm room where the attack occurred was unlocked. No weapon was used in the assault.

"We have no reason to believe the attacker was an insider, and we have no reason to believe the attacker was an outsider," HUPD Chief Francis D. "Bud" Riley said. "However, it was committed in a residence hall."

There are no suspects at this time, he said.

Because the victim filed a police report on the condition of anonymity, Riley declined to release the location or specific circumstances of the attack.

"We're trying to restrict the information as much as possible," he said.

The term sexual assault encompasses a variety of crimes from indecent assault to rape. Riley declined to comment on the nature of this assault.

The attack, which occurred around 4:30 a.m., was reported to police within an hour.

This is the first reported sexual assault of the school year. Riley said he did not know of any similar recent incidents at other Boston-area schools.

HUPD issued a community advisory on Friday morning after discussions with the victim.

"This warning is very serious," he said. "There's a security issue here."

Dean of Freshman Elizabeth Studley Nathans said the Freshman Dean's Office (FDO) is making resources available to the victim and declined further comment.

"Our first concern is for the safety and welfare of both victim and community--ensuring immediate help and ongoing support to the victim, and urging on all members of the community the importance of safety and security measures," she wrote in an e-mail message.

Police warned students to be aware of any suspicious activity in their dorms and reminded them to lock their doors.

Riley said they believe the door to the room where the attack occurred was unlocked.

"I'm really concerned about people not adhering to the two-lock policy," Riley said. "We can't overstate the need for security. We've said it before, we said it during the Yard burglar case."

Dean of the College Harry R. Lewis '68 expressed similar concerns, saying that while he supports the idea of installing self-locking doors on all dorm room, student opposition has stalled the idea.

"If the locks really improve safety, I don't mind that students find them inconvenient, but if students dislike them so much that we wind up in a perpetual battle over maintenance issues, and safety isn't improved anyway because students disable the locks, then I am not sure they are worth it," he wrote in an e-mail message.

Massachusetts law provides that reports of rape and sexual assault are not public documents. If police find a suspect in the case, they will discuss with the victim whether she would be willing to press charges.

Riley said since the attack occurred in a darkened room, police were without much information.

"We don't have a lot to go on," he said. "Most of the information with the exception of the identifiable details of her name and dorm have been released."

Anyone with information regarding the attack is asked to contact HUPD's criminal investigations unit at (617) 495-1796.

--Staff writer Garrett M. Graff can be reached at ggraff@fas.harvard.edu.