The 12-year-old Philly boy knew the man as "Bobby," a family friend. But police and the New York State Sex Offender Registry knew him as Robert Alvarez, a sexual offender required to register for life.

Now, Alvarez again faces charges that he not only sexually assaulted a young boy - but a young boy he knew.

On Saturday, police said Alvarez, 44, of Staten Island, N.Y., who was visiting the 12-year-old boy's family, pulled up to where the boy was playing basketball on Lawrence Street near Rockland in Olney around 9:30 p.m. and asked him if wanted to go buy fireworks.

The boy got into Alvarez's van and Alvarez drove nearly three miles to Rutland Street near Oxford Avenue in Frankford, where he parked the van, according to police.

A police source said residents of that area watched the van with New York plates park and witnessed as Alvarez and the boy went from the front of the van to the back.

Suspicious, the residents called police.

"This is outstanding that the neighbors got involved in this," the source said.

Police pulled up on the scene and heard the boy screaming. When they opened the van they said they found Alvarez and the boy in the back, on a mattress.

It was determined a sexual assault had taken place and Alvarez was subsequently charged with attempted rape, indecent sexual assault, false imprisonment and related offenses, police said.

According to the Megan's Law Web site, Alvarez was in a category with the worst of all sexual offenders, which meant he had a "high risk of repeat offense" and was "a threat to public safety."

Alvarez was arrested in 1994 by the New York City Police Department's Sex Offender Unit on first-degree sexual abuse charges involving an 8-year-old boy and a 10-year-old boy, according to the Web site.

The offenses occurred more than once and his victims were not strangers to him, the Web site says. He specifically "established relationship for purposes of victimization (grooming)," his profile reads.

His sentence appears to have been 4 years and 7 months to 14 years in state prison, though it's not clear from the Web site when he was released.

He was last registered in Staten Island, but the Megan's Law Web site currently lists him as "homeless."

Staff writer Michael Hinkelman contributed to this report.