The number of reported sexual assaults has dropped again at the nation's three major military academies, the Defense Department said Friday, noting that underreporting could be a reason for the decrease.

The change represents a significant decrease in reports at the U.S. Air Force Academy in Colorado Springs, Colo., but a rise at the U.S. Naval Academy in Annapolis, Md. and the U.S. Military Academy in West Point, N.Y.

The military's report said that, overall, reported sexual assaults dropped to 25 for the 12-month period ending in May 2009, compared to 34 the previous year. Forty assaults had been reported in the year ending in May 2007, the first time the Pentagon released a report on efforts to prevent sexual assault at the academies.

"While the (military service academies) as a whole have implemented policies, programs and curricula to address the problem of sexual assault, the long-term social changes enacted may not yield short-term observable results," the report released Friday said.

In last year's report, the Defense Department estimated that 90 percent of sexual assaults are going unreported based on anonymous surveys _ a percentage the department said is consistent with the underreporting trend in most civilian college populations. This year's report did not include an anonymous survey, but next year's assessment will look again at the degree of underreporting.

Now that the programs are firmly established, the Defense Department said a high priority must be placed on empirically measuring the effectiveness and outcomes of each institution's programs.

The military began allowing "restricted reporting" in 2005 to let victims to file reports without initiating an investigation or notifying their commander.

Of the 25 assaults in the most recent report, nine were reported from West Point, eight from Annapolis, and eight from Colorado Springs. The overall decrease compared to last year is the result of a significant drop at the Air Force Academy, which went from 24 reported incidents to eight. West Point had five more cases reported, and the Naval Academy had two more.

"We're not quite sure we know how to characterize the decline, but we know we want to keep the heat up on our program," said Air Force Col. Gail Colvin, the academy's vice commandant of cadets for culture and climate.

The report released Friday included results from student focus groups.

At the U.S. Military Academy, students said fear of damaging their reputation was frequently cited as a reason why an assault might go unreported.