National Park Service workers at the Grand Canyon sexually preyed on their female colleagues during isolated river trips over a period of 15 years, a federal investigation has found.

A report by the inspector general of the US Department of the Interior looked into complaints that officials mishandled charges that trip leaders pressured female co-workers for sex, touched them inappropriately, made lewd comments, and retaliated when rejected or when victims complained.

Close to a dozen men have faced disciplinary action, from reprimands to termination, for sexual misconduct against female colleagues since 2003. But a complaint filed in late 2014 by 13 current and former workers said action taken was inconsistent and inadequate.

A "laissez faire" attitude of "what happens on the river stays on the river" informed official response to complaints, one human resources official told federal investigators.

One river district employee said male employees tried to "get laid as much as possible" during river trips. There was "some kind of wager" among three men concerning which of them could get the most sex, according to one worker. One male co-worker defended two of the men as "free spirits."

Women said the men reacted in a hostile manner when rejected; one reported being denied food by her male co-worker when she spurned his sexual advances.

"We found evidence of a long-term pattern of sexual harassment and hostile work environment in the River District," federal investigators stated in their report. "In addition to the 13 original complainants, we identified another 22 individuals who reported experiencing or witnessing sexual harassment and hostile work environment. We also confirmed that some of the incidents were reported to supervisors and managers but were not properly investigated or reported."

"It was a culture of victim-blaming perpetuated by all levels of management," one woman wrote in a complaint. "I repeatedly sat in meetings in which victims who had reported sexual violence were degraded and discredited."

The Grand Canyon National Park manages 280 miles of the Colorado River in Arizona. Federal Park Service workers provide emergency and medical services, as well as guiding researchers, tourists and students on river trips. Co-workers spend long stretches together within the massive canyon, camping and isolated from the rest of the world.

The federal report does not identify any of the park employees, boatmen or workers by name, and it focuses solely on trips run by Grand Canyon National Park Service. Some specific incidents of sexual harassment on the trips mentioned include a boatman photographing an employee under her skirt and a supervisor grabbing an employee's crotch.

Park managers were aware of the history on the river, Grand Canyon officials told investigators, and said the agency tried to change the culture. Alcohol appeared to play a role in the misconduct, according to a spokeswoman.

"No National Park Service employee should ever experience the kind of behavior outlined in the report, and it is even more disappointing because previous efforts to change the culture at the River District of the grand Canyon failed to improve working conditions," spokesman James Doyle said in an email to Associated Press.

Alcohol was banned last year as the federal investigation was being conducted. A Park Service representative said a series of changes are now being considered to protect female workers, including requiring nightly check-ins by satellite telephone and including a supervisor on every river trip.