TOKYO – At U.S. military bases in Japan, most service members found culpable in sex crimes in recent years did not go to prison, according to internal Department of Defense documents. Instead, in a review of hundreds of cases filed in America's largest overseas military installation, offenders were fined, demoted, restricted to their bases or removed from the military.

In about 30 cases, a letter of reprimand was the only punishment.

More than 1,000 records, obtained by the Associated Press through the Freedom of Information Act, describe hundreds of cases in graphic detail, painting a disturbing picture of how senior American officers prosecute and punish troops accused of sex crimes. The handling of allegations verged on the chaotic, with seemingly strong cases often reduced to lesser charges. In two rape cases, commanders overruled recommendations to court-martial and dropped the charges instead.

Even when military authorities agreed a crime had been committed, the suspect was unlikely to serve time.

The analysis of the reported sex crimes, filed between 2005 and early 2013, shows a pattern of random and inconsistent judgments.

For example, in two cases, both adjudicated by the 1st Marine Aircraft Wing, the accusers said they were sexually abused after nights of heavy drinking, and both had some evidence to support their cases. One suspect was sentenced to six years in prison, but the other was confined to his base for 30 days instead of getting jail time.

Taken together, the cases illustrate how far military leaders have to go to reverse a growing number of sexual assault reports. The records also may give weight to members of Congress pushing to strip senior officers of their authority to decide whether serious crimes, including sexual assault cases, go to trial.

U.S. Sen. Kirsten Gillibrand, who leads the Senate Armed Services' personnel subcommittee, said Sunday the records are "disturbing evidence" that there are commanders who refuse to prosecute sexual assault cases.

Air Force Col. Alan Metzler, deputy director of the Defense Department's Sexual Assault Prevention and Response Office, said the department "has been very transparent that we do have a problem." He said a raft of changes in military law is creating a culture where victims trust that their allegations will be taken seriously and perpetrators will be punished.

The number of sexual assault cases taken to courts-martial has grown steadily — from 42 percent in 2009 to 68 percent in 2012, according to DOD figures. In 2012, of the 238 service members convicted, 74 percent served time.

That trend is not reflected in the Japan cases. Out of 473 sexual assault allegations within Navy and Marine Corps units, just 116, or 24 percent, ended up in courts-martial. In the Navy, one case in 2012 led to court-martial, compared to 13 in which commanders used nonjudicial penalties instead.

The authority to decide how to prosecute serious criminal allegations would be taken away from senior officers under a bill crafted by Gillibrand that is expected to come before the Senate this week. Senior U.S. military leaders oppose the plan.