Many states fall short of federal sex offender law

"They were very in tune with what requirements were in different states, and they would frequently migrate to other states," said Pursley, who is now retired.

James Womack, a convicted sex offender from Oklahoma who now works for a nonprofit agency that helps recently released felons, said he understands the need for consistent registration rules. But he cautioned that registration alone will not stop them from reoffending.

"It doesn't do anything to stop crime," said Womack, who was convicted in 2005 of indecent liberties with a child and served nearly two years in prison. "A true pedophile, if they're going to offend, they're going to offend, whether or not they live one mile or 10 miles from a school."

Sean Murphy can be reached at www.twitter.com/apseanmurphy

Associated Press writers Nomaan Merchant in Dallas, Don Thompson in Sacramento, Calif., and Bob Christie in Phoenix contributed to this report.