McCaskill released the findings Wednesday from a national campus sexual assault survey.
"I thought it was very important we take an in-depth survey of colleges in this country to find out what is the nature of this problem," said McCaskill.
The survey of 440 colleges found that more than 40 percent of schools have not conducted a single investigation in five years.
It also showed that 21 percent of schools provide no training to faculty and staff and 31 provide no training for students.
McCaskill says many universities are breaking the law in how they handle sexual violence against students.
"Perhaps, potentially the most disturbing part of this survey that we have now learned is federal law requires that every institution that knows or that reasonably should have known about sexual violence to conduct an investigation. That is black letter law in this country right now. 41 percent of the schools that were surveyed had not conducted a single investigation in the last five years," said McCaskill.