Over the course of 15 years, since about 2000, three male boat operators and a supervisor sexually harassed at least 35 female National Park Service employees during work trips on the Colorado River, according to an investigation released by the Department of the Interior on Tuesday.

The report, which found “evidence of a long-term pattern of sexual harassment and hostile work environment,” stemmed from a letter of complaint and a request for investigation sent in September 2014 to DOI Secretary Sally Jewell by 13 former and current employees of the National Park Service, all of whom had worked, at various time, in the Grand Canyon National Park’s River District.

The complainants described incidents that they said were evidence of “discrimination, retaliation, and a sexually hostile work environment” over the course of 15 years. In the course of investigating the complaint, the DOI identified 22 other individuals who described similar experiences, which included inappropriate touching and threats.

“The behavior cited in the report is simply unacceptable,” James Doyle, a National Park Service spokesman, told Reuters.

Twelve times a year, the National Park Service conducts scientific and restoration trips, which sometimes last several weeks, up the Colorado River and through the Grand Canyon. The boats and boat operators are provided through the River District.

The DOI interviewed 19 individuals who stated that River District employees had behaved inappropriately, identifying four current and former employees—who are not named in the report—who touched them inappropriately, made sexual comments, or propositioned them for sex. From the report:

  • Employee 1 described an incident in which Boatman 2 took a photograph under her dress during a 2005 river trip.
  • Employee 2 described being repeatedly propositioned for sex by Boatman 1 during river trips.
  • A former seasonal employee (Employee 3) reported that Boatman 1 repeatedly propositioned her for sex, that Boatman 3 was rude, and that Supervisor 1 had yelled at her. She said that she resigned from GRCA in 2012, after 9 years as an NPS employee, because of “undue levels of stress” caused by this treatment.
  • Another former employee, Employee 4, said that during a 2005 river trip, Boatman 3 behaved in a threatening manner toward her—yelling at her while holding an axe—while he was intoxicated.
  • Employee 5 said that Boatman 1 inappropriately touched her back and buttocks during a 2013 river trip, and that Boatman 3 yelled at and belittled her during her employment at GRCA.
  • Another former employee, Employee 6, recalled Boatman 3 asking her to give him a massage during a river trip. She said that his request made her uncomfortable and that she declined.
  • A former NPS volunteer (Employee 7) reported that Boatman 3 inappropriately touched her thigh during a 2010 river trip.

Last year, in January, the U.S. Attorney’s Office in Arizona declined to prosecute any of the individuals in the report, all but one of whom have resigned. One of the boatmen is still employed at the boat shop, Outside.com reports, but is not permitted to travel on river trips. In March, the Grand Canyon River District banned alcohol from trips for both NPS employees and commercial participants.

One of the women the DOI interviewed, a longtime Grand Canyon worker, said she had been friends with two of the accused for years, describing them as “free spirits” who told “off-color jokes.”

Photo via AP Images. Contact the author of this post: brendan.oconnor@gawker.com.