The University of Colorado Police Department has released a composite sketch of one of two suspects who may have been involved in a sexual assault on the Boulder campus early Sunday morning.

A CU student told police that at 1:30 a.m. Sunday she was walking alone on the north side of Norlin Library when she was attacked.

One man knocked her to the ground and sexually assaulted her while another man acted as a lookout, she said.

No weapons were displayed, according to the police report.

"It's unusual and it's uncommon to have this type of sexual assault for a couple of reasons," said CU police spokesman Tim McGraw. "For one, it's a stranger rape. Those are very uncommon at CU, and in Boulder in general."

He said that most rapes that occur are date-rapes or acquaintance rapes.

The victim told officers that during the attack she struck one of the suspects in the head with her belt, so one suspect may have facial injuries, police said.

"It's really sad that we get such a beautiful school in Boulder and yet people will actually go out and do something like that to someone," said student Mirabai Agle.

One suspect was described as a college-age white male with red hair about 3 inches long, wearing a dark-color shirt and jeans (pictured, above). The second suspect was described as a white male, who was shorter than the first suspect, with dark hair, and wearing a light-color shirt and khakis.

Officers are hoping to complete a composite drawing of the suspects soon. Anyone in the area at the time or who may have more information is asked to contact Detective Brannon Winn at (303) 492-8168.