"On first blush, a parent would think that's good, they don't have a problem with sexual assault on their campus, but it's not good, it's very bad because that means they are either in denial or incompetent," McCaskill said.
Federal law requires every institution that knows about a sexual violence incident to investigate, she noted. She said schools should investigate even if the end result is that the victim isn't participating and there's no corroboration. Under some estimates, 1 in 5 college females is assaulted.
The prevalence of sexual assaults on college campuses took on new focus in the wake of the Jerry Sandusky case at Penn State and after a high-profile battle on Capitol Hill about military sexual assault led college campus assault victims to demand the same attention.
Meanwhile, the Education Department and a White House task force on campus sexual assault have taken a series of steps to draw attention to the treatment of sexual assault victims and force campuses to address the problem.
In Congress, McCaskill is part of a group of senators exploring ways to address the issue legislatively. She said the survey was needed so they had a better grasp of how campuses handle such cases.
McCaskill said the senators are looking at ways to empower victims, simplify laws and rules colleges and universities follow and find ways that campuses and local authorities can better coordinate. She chairs a subcommittee with jurisdiction over Title IX, which prohibits gender discrimination at institutions receiving federal funds.
Ada Meloy, general counsel of the American Council on Education, which represents college presidents, said if victims want to maintain confidentiality, it is "extremely difficult to conduct an investigation." She said many college officials want to work more with local authorities, but local authorities are hesitant to take such cases because they are difficult to successfully prosecute.
Meloy said her organization is disappointed by the report and says it fails to describe how hard colleges and universities are working to address the problem under a complex and confusing set of federal guidelines and laws.
About 40 percent of schools said they have sworn law enforcement officers on campus, while many others have private security and about half rely on local authorities. Thirty percent said campus police and security guards aren't required by law or institutional policy to be trained to respond to reports of sexual violence.
Only about a quarter of the schools said they have written protocols between campus and local authorities for handling such cases.
Most schools said they use a "team" response to reports of sexual assault, but only about a quarter incorporate the local prosecutor's office on the team.
Among the other findings:
More than 20 percent of respondents provide no sexual assault training for all faculty and staff.
More than 30 percent of schools do not provide sexual assault training for students.
About half of the participating colleges and universities do not provide a hotline for sexual assault victims.
About 16 percent of respondents conduct "climate surveys" to gauge the number of such cases that are going unreported.
About 10 percent said they don't have a Title IX coordinator.
"Many institutions continually violate the law and fail to follow best practices in how they handle sexual violence," McCaskill said.
The findings come from a survey of 440 four-year colleges and universities of different sizes with 236 colleges and universities responding. Participating schools weren't named.