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(CBSNews.com) - In the wake of troubling revelations about the handling of sexual assault charges on U.S. military bases in Japan, Sen. Kirsten Gillibrand, D-N.Y., is calling on the Pentagon to hand over detailed information about sexual assault cases at major U.S. military bases.

Gillibrand, the chief sponsor of a bill to combat sexual assault in the military, sent a letter Monday to Defense Secretary Chuck Hagel, demanding information pertaining to reports of sexual assault and related offenses from January 1, 2009 through January 1, 2014 at Fort Hood, Naval Station Norfolk, Marine Corps Base Camp Pendleton, and Wright-Patterson Air Force Base.

Gillibrand's spokesperson, Glen Caplin, said in a statement that the senator has a "responsibility" to review the data in light of documents revealing the chaotic way the U.S. military handled sexual assault cases in Japan. The documents,obtained by the Associated Pressthrough a Freedom of Information Act request, shed light on more than 1,000 reports of sex crimes involving U.S. military personnel based in Japan from 2005 through early 2013.

"As the Chair of the Armed Services Personnel Subcommittee, it would be irresponsible [for the senator] to assume or conclude the 'chaotic' nature of 'random and inconsistent judgments' in Japan is an anomaly, a further review of data from additional bases is essential to provide transparency and accountability," Caplin said in a statement to CBS News.

The AP report showed that charges of rape were often reduced, even in cases with strong evidence. Of the 244 servicemembers whose punishments were detailed in the records, nearly two-thirds were not incarcerated.

"Taken together, the sex crime cases from Japan, home to the largest number of U.S. military personnel based overseas, illustrate how far military leaders have to go to reverse a spiraling number of sexual assault reports," the AP reported.

Gillibrand's new request for information from the Pentagon comes ahead of an expected vote this week on her bill, the Military Justice Improvement Act. The legislation would give military prosecutors instead of commanders the authority to prosecute sexual assault crimes -- taking the decision out of the chain of command and thereby encouraging victims to come forward with less fear of retribution from their commanders.

Gillibrand said last week she believes her bill has the 60 votes necessary to overcome a filibuster, even though the legislation hassplit the Democratic party. Last week, Sen. Claire McCaskill, D-Mo.,said she would support a filibusteragainst Gillibrand's bill.

"We do not believe that her bill will protect victims," McCaskill said. "We do not believe that it will result in more prosecutions. We do not believe it will increase reporting. And we believe there's real workability problems in terms of how quickly these cases will be brought to court."

According to estimates from the Pentagon,as many as 26,000 service members suffered a sexual assaultin 2012. Hagel has worked to improve accountability and improve victims' rights, the report says, and has been instructed by the president to continue focusing on the issue and report back on progress by December 2014. Thedefense authorization billthat President Obama signed in December took some steps to to reform the way the military prosecutes sexual assault cases, but lawmakers did not get the chance to vote on whether to include Gillibrand's proposal.

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