Each year local law enforcement agencies decide which rape cases will be reported to the FBI and which ones will not, sometimes leaving rapists on the streets to attack again.

A national expert says the number of rapes reported often are underreported in midsize to larger cities like Chattanooga so the numbers don't look as bad.

Cases are labeled unfounded when officers investigate a complaint and determine that a crime never happened.

READ MORE | More than 250 sexual assault kits never sent to crime lab

“There's some body of complaints that are unfounded. I think they typically revolve around false reports when someone is mentally ill, seeking attention,” Corey Rayburn Yung, an associate law professor at the University of Kansas says in an interview with Channel 3.

In Chattanooga, a total of 35 percent of rape cases were considered unfounded last year. In 2013, nearly half of all reported rapes were labeled unfounded, according to statistics kept by the department.

“If a case gets unfounded on front end and the case doesn't go anywhere -- it's determined that the … accusation is false,” says Chattanooga Police Lt. Glenn Scruggs, who oversees the violent crimes unit. “That [sexual assault] kit doesn't get submitted [to the lab]. It happens a lot.”

Those numbers are high compared to the national average.

Nationally, the average number of unfounded rape cases is between 2 and 5 percent.

“So, you shouldn't have a very high number of unfounded. That would be anomalous,” Yung says.

Chattanooga police say they investigate all rape cases when someone files a report.

“We leave every case open until a successful arrest is made or the other conclusion which could be it's unfounded, the suspect isn't the right guy or the story isn't what we were led to believe on the front end,” Scruggs says.

Departments select unfounded cases that are then subtracted from the total number of reported rapes. Those totals are used to calculate violent crime statistics.

“The idea of designating cases unfounded is part of the language the FBI set up to try and make sure the measured levels of crime on a national level were not including false reports,” Yung says.

Other crimes are rarely designated at high levels as unfounded even though there might be false reporting.

“For example, theft cases. People who report a fake theft as a result of an insurance fraud. You report something stolen and then you collect insurance money,” Yung says. “But with rape cases, it has become a tool by departments who have become very motivated for statistical goals.”

In some police departments with high numbers of unfounded cases, the label has turned into a dumping ground -- a way to dispose of cases.

“They can make it look like crime statistics are better and that crime rates are down without having engaged in any investigation or done anything to make the public more safe,” he says.

Channel 3 recently completed an investigation looking at the issue of untested sexual assault kits never sent off for testing at the state crime lab.

The Chattanooga Police Department and the Cleveland Police Department reported many of the untested kits were from unfounded cases.

Chattanooga police reported 54 percent of the untested kits were from unfounded cases. Cleveland reported 33 percent. The Hamilton County Sheriff's Office never responded to a request showing breakdown of cases.

Even with more awareness of backlog kits sitting untested in evidence rooms, local police departments say they have no plans to test kits from unfounded cases. The sheriff's office has destroyed most of its sexual assault kits and plans to destroy more, citing a court order. When asked, they did not explain why.

Historically in many police departments, there has been a culture of not believing rape victims, Yung says.

Part of it is institutional and part of goes back to the way officers are trained.

“They lack the empathy that's often needed ... And so cases get ignored,” he says. “If drugs are involved, if prostitution is involved. ... Cases are just instantly discarded in many jurisdictions.”

Yung conducted a study using FBI uniform crime reporting statistics from 1995 to 2012 in cities across the country, including Chattanooga.

He compared the number of homicides reported to the number of rapes.

In Chattanooga, there has been a steady decline in reported rapes since 2005. That trend occurred while the city's murder rate increased, Yung says.

“That's a bad sign. Normally those things go hand in hand,” he says. “If that continues, it would make me think rape cases are not being investigated well.”

Last year's rapes nearly doubled to 83 cases.

“No, I can't think of a clear reason why more were reported. Maybe because there's more faith in justice being served. I mean you know if …. I'm sure that at one point there was the feeling of, 'Hey I don't want to go through the trauma of it. I don't want to go through the pain of a trial,'” Scruggs says. “I think folks have more faith in our department and law enforcement period as far as reporting these cases.”

However, it should be noted in that same year, an additional 35 percent of cases were ruled unfounded and removed from the total count.

Rape cases require a lot time and resources, Scruggs says.

“Rapes are challenging from the start. You know, of course you get a ton of rapes which aren't the ones you see on television where there's some stranger lurking in the parking garage. And then some woman walks to her car and gets raped,” Scruggs says. “Those are the outliers. The majority of the rapes are acquaintances.”

Scruggs says those are hard to prove. A suspect may admit to having sex with the victim. The suspect also will say it was consensual.

“The hardest part about that case was proving that it wasn't,” he says. “A lot of those get lost to the system. A majority are ‘he said, she said.'”

The same detective who investigate rapes at the department investigate other violent crimes including homicides and aggravated assaults. On average, they take on five new cases per month to add to 30 to 40 active cases.

Even in cases that detectives actively pursue, evidence is sometimes not immediately sent off for testing.

Police have to drive to Nashville and Knoxville to deliver evidence because the chain of custody cannot be broken.

Rather than make frequent trips to labs, investigators often wait until they have enough evidence to justify a trip. Sometimes that takes about a month. Detectives have the option of putting a rush on a kit.

Next month, the department will start a new unit called the Special Victims Unit, which will have the ability to focus on rape cases. That unit officially begins June 8.

“These guys will have a laser beam focused on [rape cases,]” Scruggs says. “It will be their primary task. So, I think you'll probably get better service when it's whittled down to one group of guys working these cases.”

Yung says units targeting sex crimes in other cities sometimes still have issues with unfounded cases. Coupled with a new state law going into effect July 2016, kits will have to be tested within 60 days. However, whether a case should be investigated and whether a kit should be sent off is still left at the discretion of the department.

“Given that there's been an overall reluctance by police departments to fully investigate rapes because they are often resource and time intensive, they are hard cases to bring to trial. I think it's just better to err on the safe side and test everything,” Yung says. “The simple fact is, as jurisdictions have started testing these kits, they have gotten hits for people who were in the system who had other convictions for sexual violence.”