BOULDER COUNTY, Colo. (KDVR) — The Boulder County Sheriff’s Office says a woman was sexually assaulted while hiking on Walker Loop Trail.

The sheriff’s office said the attack occurred Tuesday afternoon near 7701 Flagstaff Rd., about 9 miles west of Boulder.

The woman was hiking alone when she says she was approached by an unknown man who sexually assaulted her.

“He continued to be very aggressive, grabbed onto her, grabbed around her body, and she was trying to fight him off. She was screaming and eventually he let go, and they both went separate ways,” said Det. Sgt. Brian Lindsey with the sheriff’s office.

The suspect is described as a white man in his 40s. He is “tall and thin, yet muscular.” He is bald and wore a mask during the incident, according to the sheriff’s office.

“The trails are very busy, so I suspect others have had an encounter with this individual, and we would like to know that. We have no idea who this person is,” Lindsey said.

If you were hiking in that area on Tuesday and you may have witnessed anything suspicious, please contact Det. Asa Merriam at 303-579-3611 or email at amerriam@bouldercounty.org.

“This is a reminder that you should always be diligent and aware of your surroundings when recreating, especially if you are alone,” the sheriff’s office said in a statement.