TALLAHASSEE, Fla. -

More than 650 men being held in a state prison are so dangerous, a judge has determined they can't be released even though they have served their sentences.

They are all sex offenders deemed likely to offend again.

A new study aims to help prosecutors better predict who might be dangerous in the future.

There are currently 657 sex offenders still locked up in Florida even though they completed their sentences. The law keeping them in jail indefinitely was passed in 1998 after the brutal rape and murder of 9-year-old Jimmy Ryce.

"He was a very smart, perfect child," said Donald Ryce, Jimmy's father.

Named for boy, the Jimmy Ryce Act allows those whom psychologists deem still dangerous to be involuntarily committed.

"There is a few people at the facility who have been there ever since the beginning of the program in 1999," said David Monfaldi, administrator for the sexually violent predator program. "Other people stay for varying lengths of time."

Donald Smith, the man accused of killing 8-year-old Cherish Perrywinkle, was committed under the Jimmy Ryce Act in 1999, but prosecutors didn't pursue permanent lockup. Now a new study of sex offenders in Florida shows what many already know: Offenders, like Smith, are incurable.

"After 10 years, nearly 14 percent of the sample had been rearrested for sexual crimes," said Jill Levenson, of Lynn University.

Smith was arrested again in 2009 for a sex crime, but because the charge was knocked down to a misdemeanor, a risk assessment wasn't triggered.

"When we are making assessments solely on the offense of conviction, here's an example of where that criteria can really underestimate the risk of somebody's likelihood to commit a future violent offense," Levenson said.

The hope for the new study is to better predict who will be dangerous in the future.

For tips on keeping your children safe, visit JimmyRyce.org. The site offers tips, called Great Escape Maneuvers, or GEMS, for children in difficult situations.