Dozens of colleges around the country are not investigating sexual assault cases on campus, and a significant number are giving oversight of incidents involving athletes to school athletic departments, according to a major survey released Wednesday.

Sen. Claire McCaskill (D-Mo.), the chair of the Subcommittee on Financial and Contracting Oversight, commissioned the report, which is the first of its kind. The survey was distributed to 350 colleges and universities , and the subcommittee received 236 responses from a variety of schools across the country.

More than 40 percent of the schools surveyed said they have not conducted a single sexual assault investigation in the past five years. That total includes 6 percent of the large public universities that participated in the survey.

The national sampling also found that more than 20 percent of schools give oversight of sexual violence cases involving athletes to their university's athletic department. This method of oversight was higher at Division II and III schools than Division I.

McCaskill theorized athletic department oversight of sexual assault cases "is probably just something that has been allowed to exist because no one was paying close attention."

"On some campuses athletic departments are the most powerful entity," McCaskill told The Huffington Post. "I intend on asking the head of NCAA about it this afternoon."

Nine percent of schools in the sample reported conducting fewer investigations of sex offenses in the past five years than the total number of incidents they reported to the U.S. Department of Education in annual Clery Act disclosures. The Clery Act requires colleges to document and disclose all reported sexual assaults and other crimes on campus, and the gender equity law Title IX requires schools to respond to instances of sexual violence on campus.

The survey found that 21 percent of the nation's largest private institutions conducted fewer investigations than the number of incidents reported, "with some institutions reporting as many as seven times more incidents of sexual violence than they have investigated."

S. Daniel Carter, the director of the 32 National Campus Safety Initiative at VTV Family Outreach Foundation, a campus safety group started by the families of the victims and survivors in the Virginia Tech shooting, said this discrepancy could depend on whether victims feel they can pursue their case.

"Previous research has shown that there are significant barriers for the survivors of campus community sexual violence to make an official complaint or pursue an investigation, so that there are fewer investigations than statistics is not surprising," Carter told HuffPost.

Carter said the Campus SaVE Act , a portion of the Violence Against Women Act that adds new requirements on how colleges must conduct campus disciplinary proceedings for sexual violence cases, will help "empower more survivors to get justice if they elect to pursue it." Congress passed the latest reauthorization of VAWA in February 2013.

McCaskill's office is declining to release individual university responses "in order to obtain candid feedback from each school." Instead, she is opting to release responses based on institution size and athletic division, and compare answers from public and private schools.

The questionnaire, which faced some backlash from higher education groups, used 28 questions that came from a 2002 National Institute of Justice report on how colleges were handling sexual assault cases on campus.

The schools sampled vary in size, and include private and public institutions. They collectively educate more than 3 million students, the report said.

Here are some additional highlights from the survey's findings:

• Just 44 percent of institutions in the national sample provide the option to report sexual assaults online. Approximately 8 percent of institutions still do not allow confidential reporting, down from 16 percent identified in the 2002 NIJ report.

McCaskill is planning to unveil legislation to address campus sexual assault later this summer, along with Sens. Richard Blumenthal (D-Conn.) and Kirsten Gillibrand (D-N.Y.). The senators told reporters last month they expect the bill to have bipartisan support, and McCaskill said Wednesday the legislation will likely be unveiled in August or September.