OLYMPIA - A registered sex offender and an online ad for babysitting make for a volatile mix. Now that sex offender is back in jail in Olympia facing new charges.

For months, registered sex offender John Gilbert Gray checked in at the Thurston County Sheriff's Office saying he was homeless. But prosecutors say it was a ruse, so police wouldn't know where Gray was and his neighbors wouldn't know what he was.

It all came to a crashing halt when a savvy mom tracked Gray on the state's sex offender Web site and put two and two together.

Authorities say Gray had been living in an Olympia apartment complex with his wife, but registering with the Sheriff's Office as homeless with no address.

"And he did admit to law enforcement," says Deputy Prosecuting Attorney Jodilyn Erickson-Muldrew, "that the reason why he was doing the transient status was because the notification would have gone out to the neighborhood."

But a young mother in Gray's neighborhood, who fears retaliation and wants to remain anonymous, says she was checking Craigslist, and found an ad for babysitting that made her suspicious.

"We decided out of the blue to check it up on Washington State sex offender's list and lo and behold there was his face," the mother said.

In the neighborhood, the woman says Gray went by the name "Logan". She says she and other mothers were alarmed to realize Gray is a level three-sex offender -- the highest risk to re-offend -- and that he and his wife were advertising to babysit.

So she reported him.

"We're all at risk with a sex offender on the loose," says the woman and, "especially one's not telling the truth as to who he is and why he's there and what he's doing."

Gray was originally convicted of assaulting a 21-year-old woman, and while there may not be any legal reason to prevent him babysitting, Thurston County is investigating.

"It's still concerning just that he is a level three sex offender," adds Erickson-Muldrew.

For the moment, prosecutors have charged Gray with violating the state sex offender registration law; if convicted he could face up to 12 months in jail. He is currently being held in lieu of a $10,000 bond.