Investigators in Houston have completed the testing of more than 6,600 unprocessed rape kits, an undertaking that resulted in 850 DNA hits in the national FBI database and could lead to rapists being put behind bars, officials announced Monday. But across the U.S., possibly hundreds of thousands of rape kits, some dating back decades, remain in law enforcement storage or in crime lab warehouses, according to the Department of Justice.

"This is not a Houston problem. It's not a Texas problem. It's a nationwide issue that built up over years and years," Houston Mayor Annise Parker said during a news conference Monday, according to the Associated Press. The city set out to handle its rape kit inventory, which dates back nearly 30 years, in 2013. Investigators completed the testing in the fall.

The exact number of untested rape kits nationwide cannot be determined, but previous investigations have identified cities with thousands of unprocessed rape kits in storage. Memphis, Tennessee, has recently faced lawsuits from rape victims urging the city to test its 12,000 unprocessed kits. Last year, Cleveland began processing thousands of untested rape kits, according to NPR.

Cities began turning their attention to reducing their rape kit backlogs in the 1990s after New York City started clearing its cache of more than 17,000 rape kits between 1999 and 2003, according to the BBC. In 2009, police in Detroit discovered a storage warehouse containing more than 11,000 untested rape kits. In January, the city completed processing 2,000 of those kits and has plans to test 8,000 more. So far, the processing has produced more than 750 DNA matches resulting in 14 convictions so far, the BBC reported.  

There are an estimated 293,000 instances of sexual assault every year in the U.S., according to the Rape, Abuse and Incest National Network. Nearly half of all rape victims are under 18 years of age. Eighty percent of them are under the age of 30. The majority of sexual assaults go unreported to authorities and 98 percent of all rapists never go to jail or prison, the anti-sexual assault group found.