About one in five colleges surveyed nationwide give their athletic departments oversight over cases of sexual violence involving student athletes, a Senate Democrat reported Wednesday.

That finding emerged from a survey of hundreds of colleges that Sen. Claire McCaskill’s staff conducted about campus sex assaults. The survey provides an unusually detailed look at variations in policies and customs in what has become an explosive issue for higher education.

McCaskill (D-Mo.) said she was struck by the finding that 22 percent of a national sample of schools responding to her survey gave athletic departments an oversight role in cases involving athletes.

“I don’t need to explain why that is a big problem,” McCaskill said. The departments, she said, want to protect athletes. That could create the appearance of a conflict of interest, she said, deterring those who might report an assault. “I think it would scare just about any victim into the shadows,” she said.

Student athletes have figured in some recent high-profile cases of alleged sexual assault, including at Florida State University and Vanderbilt University.

Sen. Claire McCaskill (D-Mo.), a former prosecutor, has been prominent in congressional debate on sex assault in the military and at colleges. (J. Scott Applewhite/AP)

McCaskill, a former prosecutor, has been prominent in congressional debate on sexual assault in the military and at colleges. Her staff sent questions to 350 four-year schools of various sizes and types in the nationwide sample and also to the 50 largest public universities and 40 private, nonprofit universities with at least 15,000 students each.

From the national sample, 67 percent responded. For the largest public schools, the response rate was 98 percent. For the large private schools, the rate was 85 percent.

McCaskill’s report is likely to be the prelude to legislation on the issue. The Obama administration also is ramping up oversight of colleges. More than 60 colleges and universities face federal investigations for possible violations of the 1972 antidiscrimination law known as Title IX related to their handling of sexual violence complaints.

A Washington Post analysis of federal campus-safety data from 2010 to 2012 found a rising number of reports of forcible-sex offenses at many schools.

Ada Meloy, general counsel of the American Council on Education, which represents colleges and universities, called McCaskill’s report unfair to many schools making an honest effort to address sex assault. “Everything is so negatively put,” she said. “It’s really disappointing. It doesn’t recognize how these cases are in­cred­ibly difficult to investigate or resolve.”

But McCaskill raised questions about how seriously schools take the issue.

Among her findings:

●More than 20 percent of schools in the national sample provided no sexual-assault response training for faculty and staff. More than 30 percent provided no training for students.

●More than 40 percent of schools in the national sample have not conducted a single sexual-violence investigation in the past five years.

●More than 40 percent of the largest public schools allow students to help adjudicate sexual-assault cases — a practice “rife with all kinds of potential problems,” McCaskill said, especially if students help judge peers whom they might encounter on campus.

●Sixteen percent of schools in the national sample conduct campus “climate surveys” that a White House task force has recommended as a key measure to determine the prevalence of sexual assault on campus. Experts say these surveys turn up essential information because students often do not report the crime to authorities.

A key question is what standard schools use in internal investigations to determine whether a student is responsible for sexual misconduct. The federal government says schools should use a standard called “preponderance of the evidence,” which requires a finding that it is more likely than not that an offense occurred for a student to be found responsible.

But 15 percent of schools in the national sample said in the survey that they use a higher standard to find a student responsible for an offense — mainly “clear and convincing evidence.” A few use “beyond a reasonable doubt.”

McCaskill favors legislation to require the preponderance standard. Skeptics say the preponderance standard tramples on the rights of students who are accused.