Princeton University violated a federal antidiscrimination law by not "promptly and equitably" responding to complaints of sexual violence, in one case allowing a sexually hostile environment to continue for one student, the U.S. Department of Education announced Wednesday.

The university formalized an agreement Oct. 12 with the department that includes revising policies, using a "preponderance of the evidence" standard in investigating complaints, and reexamining all complaints filed from the 2011-12 academic year through Sept. 1. It had begun rolling out new policies and procedures this year, which the Education Department said address the Title IX violations.

The changes are "intended to achieve full compliance," Princeton said in a statement. The changes, along with other actions in the agreement, will be monitored by the Education Department.

"I applaud Princeton University for its commitment to ensuring a community-wide culture of prevention, support, and safety for its students, staff, and community," Catherine E. Lhamon, assistant secretary for civil rights at the Education Department, said in Wednesday's release.

Princeton is not the only university to settle with the Education Department in recent years, as national attention has focused on sexual harassment and violence on college campuses. In 2011, the University of Notre Dame agreed to revise its policies to become compliant with the department's standards. Other high-profile agreements include Yale University in 2012 and the State University of New York system in 2013.

"This is really the regulators saying, 'Make sure your policies are in place and are compliant with the regulations now,' " said Kevin E. Raphael, a partner of Pietragallo Gordon Alfano Bosick & Raspanti L.L.P., who prosecuted sex crimes as an assistant district attorney in Philadelphia.

The investigation by the Department of Education's Office for Civil Rights found Princeton to be in violation of Title IX of the Education Amendments of 1972. Inder the terms of the deal, the university's agreement does not count as an admission of noncompliance.

"We are pleased that this investigation has been concluded with an agreement that brings the university's policies and procedures into compliance with all statutory and regulatory requirements," Christopher L. Eisgruber, Princeton's president, said in a statement.

The investigation stemmed from Title IX complaints by three people who accused the university of failing to respond appropriately to specific sexual assault complaints made by three students in the 2009-10 and 2010-11 academic years, according to a letter sent from the director of the department's New York office to Eisgruber. The Education Department opened its investigation in December 2010.

In addition to going through documentation and interviewing the students and Princeton staff and administrators, investigators conducted on-site visits to review files and recordings, according to the letter.

They found that Princeton's grievance procedures at the time violated the federal law that in part requires the schools to provide for "prompt and equitable resolution" of complaints of actions including sex discrimination, sexual harassment, and sexual assault or violence.

In the case of one student, "this failure allowed for the continuation of a hostile environment that limited and denied her access to the education opportunities at the university," the letter said.

Among the issues, the department said: The university used a "clear and persuasive" standard of evidence rather than a "preponderance of the evidence" standard, its policies did not explicitly give accusers equal opportunity as accused students to provide evidence and present witnesses, and only people found in violation of the university's policies would have a right to appeal.

In addition, Princeton's policies did not include a provision to notify a complainant of the right to proceed with a criminal investigation and Title IX complaint at the same time, the letter said.

Further, none of the school's relevant policies "provided for an assurance that the university would take steps to prevent further harassment or remedy its effects, if appropriate," the letter reads.

In May, days after a report from the White House Task Force to Protect Students From Sexual Assault, the Department of Education released a list of 55 schools it was investigating for Title IX violations regarding sexual violence.

Princeton was the only New Jersey school on that list. Pennsylvania schools under investigation are Temple University, Pennsylvania State University, Swarthmore College, Franklin and Marshall College, and Carnegie Mellon University. Princeton and other schools have since been removed from the list, and new investigations have been added.

There are now 86 schools being investigated, including Emory University, the University of Chicago, Harvard University, Johns Hopkins University, Dartmouth College, and Brown University.

One reason colleges and universities have gotten in trouble, Raphael said, is that regulations and guidances have been evolving constantly. One of Princeton's violations is a good example, he said: Using a "preponderance of evidence" standard was suggested as best practice several years ago but did not become mandatory until this year, so it would not have been the requirement at the time of the complaints that triggered the investigation.

After reviewing the documents related to Princeton's violations, Raphael said, school officials "weren't perfect . . . but they were acting and doing their best, compared to some other examples where universities were trying to conceal, hide, or not take any action whatsoever."