A baby sitter is facing charges after a mother in Longmont told police she came home from work and found him sexually assaulting her 9-year-old son, authorities said today.

Cmdr. Craig Earhart said 62-year-old Frank James Gutierrez was arrested about 10:30 p.m. Monday. He is being charged with sexual assault on a child by a person in a position of trust, a class 3 felony.

Police said the mother found Gutierrez and her son in the livingroom of her apartment earlier Monday.

He said the mother and Gutierrez are neighbors and she had entrusted him to look after her child while she was at work.

Earhart said the man had been baby-sitting the boy for a couple of weeks.

Police said Gutierrez is a registered sex offender and has been listed with the Longmont Police Department since November 2005.

He has kept his registration current, Earhart said.

Gutierrez was convicted in San Diego of sexually assaulting a 9-year-old girl and was ordered to register as a sex offender there.

Earhart said parents can visit their local police station to see if a potential baby sitter or anyone who looks after their child is listed on the sexual offender registry.

"Most agencies have them on their Web sites," Earhart said.

"Trust your instincts and don't be afraid to ask questions," he said. "Be very suspicious of odd behavior or if special attention is made toward your child that you don't think is appropriate," he said.

Earhart said pedophiles tend to gain the trust of children and their mothers or fathers by befriending them.

"Most of them go through a grooming process, where they pay close attention and befriend a child along with the parents," Earhart said.

Gutierrez is being held at the Boulder County Jail.

Earhart said parents who believe their child had contact with Gutierrez should question their children, Earhart said.

Parents should call the Longmont Police Department detective division at 303-651-8523 if they suspect their child may have been a victim.