Colorado University campus police confirmed they are investigating the report and are interviewing six Buffaloes football players and about 15 high school recruits who attended the off-campus party.
Colorado University Police Lt. Michell Irving said Thursday that campus police are working with Boulder police on the investigation, but declined further comment.
No arrests have been made in the case and no charges have been filed. Police won't say if any of the players are suspects in the alleged sexual assault.
According to a statement released by the university, Colorado coach Gary Barnett and athletics director Dick Tharp "pledged to investigate the allegation that football players and recruits were involved, and, if corroborated, they will take appropriate disciplinary action against any involved players, and will determine if there are any team violations to deal with. The (university) chancellor has confidence in Gary Barnett, his integrity, and his management of his program."
Barnett and CU Chancellor Richard Byyny said in separate news conferences Thursday afternoon that the allegations were being investigated and appropriate action will be taken.
"We're conducting an internal investigation," Barnett said. "Once the processes are over with, we'll do the appropriate thing."
Byyny said he didn't know all the details of the allegations. "What I can tell you is that our campus does not permit or condone underage drinking or sexual assault, certainly, or any sexual misconduct," he said.
The party reportedly was held off-campus Friday and was attended by some CU football players and high school recruits, The Daily Camera newspaper reported Thursday.
"The witness was somewhat reluctant in the beginning, but has since decided to pursue charges," Boulder Police Detective Sgt. Kurt Weiler said.
The party was organized by two female Colorado students who talked about the alleged assault with the Daily Camera newspaper.
They told the newspaper that everyone at the party was drinking heavily and the victim was one of the hostesses. The two students said the victim was raped by several of those attending and was taken to Boulder Community Hospital, which didn't notify police.
Hospital officials said they aren't required by law to report such assaults to authorities.
"There's no way the alleged actions of a handful of people should be used to judge the kind of players and the kind of character this team has," Barnett said. "It would be very unfair to do that."
The Associated Press contributed to this report.