Reports of sex offenses on college campuses provided to the Education Department have nearly doubled over a five-year period.

In a letter to Sen. Barbara Boxer, D-Calif., released Tuesday by her office, department officials said they believe the increase is the result of federal enforcement efforts and growing public attention to the issues of campus sexual assault.

In 2009, there were 3,357 such sex offense cases reported to the department in annual crime statistics reports, compared to 6,073 cases in 2013, the letter said.

Education and law enforcement officials say sex offenses are underreported crimes, and the true number of such cases is likely much higher.

The Obama administration has taken several steps to push colleges and universities to better tackle the problem of sexual assault, including releasing the names of colleges and universities facing Title IX investigations for their handling of such cases.

Laura Palumbo, prevention campaign specialist with the National Sexual Violence Resource Center, said the difference in the number of sex offense reports likely reflects that campuses are doing a better job following procedures and collecting and reporting such crimes. However, Palumbo said, campuses are still inconsistent in how they collect and report sexual offenses, and there have been complaints that they are underreporting the problem.

The letter was released by Boxer along with Sens. Kirsten Gillibrand of New York and Tim Kaine of Virginia. Boxer said the data makes clear why the department "must step up its efforts to address the epidemic of sexual assault on college campuses, and why Congress must ensure it has the resources it needs to protect students."

The letter was signed by Catherine Lhamon, assistant secretary for civil rights at the Education Dpeartment, and James Runcie, chief operating officer for Federal Student Aid. The two said department officials expect the average amount of time to resolve cases will decrease as the agency closes its older cases or Congress increases funding to handle the cases.

Among the other figures released in the letter:

— The department's Office of Civil Rights saw an increase in sexual violence complaints involving colleges and universities that went from 9 in 2009 to 102 in 2014.

— Cases that resulted in "substantive closures" took, on average, 1,469 days in the 2014 budget year — or about four years — for the department or resolve.