Authorities in Florida and California continued Thursday to search for two teenage girls who disappeared in separate incidents. In Florida, part of the investigation is focusing on the actions of a convicted sex offender.

In the Florida case, 200 police and volunteers looked for Sarah Michelle Lunde (search), 13, who disappeared over the weekend from her family's mobile home in Ruskin, south of Tampa.

As part of the investigation, officials questioned a convicted sex offender who once had a relationship with the girl's mother and suddenly reappeared at the family's home the day the teen disappeared.

Hillsborough County Sheriff David Gee wouldn't call David Onstott (search) a suspect in Sarah's disappearance, but said Wednesday, "Obviously, this guy has our attention and we're very interested in him. But we're looking at everyone very closely."

Mark Lunsford (search), a Florida man whose daughter's disappearance in late February also launched a full-scale search around his home, was volunteering in the search for Lunde. Jessica Lunsford, 9, was raped and killed by a registered sex offender just 150 yards from the family's home.

"It's just a missing child and they need help finding her," Mark Lunsford said, when asked if he knew or had spoken to the girl's family. "If her family needs someone to talk to who understands, I'm here for them."

Sarah, who has a history of running away, hasn't been seen since about 9 p.m. EDT Saturday, shortly after she returned home from a church trip. She wasn't reported missing until Monday, when her mother learned the teen wasn't at a friend's house.

Her 17-year-old brother, Andrew, said Onstott, 36, showed up at the family's home before dawn Sunday and asked for the teens' mother, whom he'd broken up with months earlier.

Andrew Lunde said that when he told Onstott that his mother wasn't home, Onstott grabbed a half-empty bottle of beer from the kitchen and left. Lunde said the beer hadn't been there at midnight when he left for a few hours to get something to eat.

Lunde said that when he came home at 4 a.m., the front door was open and his sister was gone. Onstott woke him up about an hour later.

"He just showed up out of the blue," Lunde said. "It was strange."

Onstott was arrested Tuesday after he allegedly threatened another man with a screwdriver at a bar. He spent 5½ years in prison after being convicted in 1995 of sexual battery with physical force. The victim, an acquaintance, testified Onstott knocked on her door, asked to use the bathroom, then threw her on the floor and raped her.

About two dozen registered sex offenders live in the immediate area. Following his personal tragedy, Lunsford has become a vocal advocate for sex offender registry reforms.

"There should be a federal database where you can just punch in your zip code that the FBI's got, that everybody's got, all on one Web site," he said.

David Onstott Sr. said he doesn't think his son is involved in Sarah's disappearance.

"I can't believe he would hurt a young girl," he said.

The sheriff's office also interviewed the girl's father, who lives in Zephyrhills, about 50 miles north of the Tampa Bay-area town. He is not considered a suspect, Gee said.

Sarah is 5-feet-2 and 170 pounds, with brown hair below her shoulders and hazel eyes. She was last seen wearing a white T-shirt and orange and blue basketball shorts. She has a bright green half cast on her left arm from a wrist injury.

Anyone with information about her is asked to call the Hillsborough County Sheriff's Office (search) at (813) 247-0455.

Search Continues in California

Across the country in California, authorities are looking for Ashleigh Markley (search), a 14-year-old who disappeared on March 29 as she was visiting her grandmother and other relatives in Lincoln, Calif.

Lincoln police initially considered Ashleigh to be a runaway but the Citrus Heights Police Department (search), which took over the case last week, considers Markley missing and at risk. Ashleigh lives in Citrus Heights, a community near Sacramento.

Officials think Ashleigh may be with Lindsay Jones, 28, who the teenager met during an Easter event at her grandmother’s house. Jones is also known as Lindsay Rehders.

Police urged Jones to contact them and tell them what she knew of Ashleigh's whereabouts, stressing that if the teen was with her voluntarily she would face no charges.

Ashleigh's parents appealed for her safe return on FOX News on Thursday morning.

"Tell her I love her and not to be afraid and come home. She needs to be with us, not with that lady," said her mother, Tammy Markley Barnes. "I don't know what kind of situation she's in. I don't know if she's befriended her. I don't know anything."

The girl's stepfather, Kevin Barnes, said they don't feel she ran away this time because Ashleigh hasn't called in. "We just want our daughter home. We miss her and love her," he said.

Ashleigh is 4-foot-11 inches and weighs 86 pounds, with blue eyes, blond hair and piercings in her tongue and navel. Anyone with information about her is asked to call Citrus Heights police at (916) 568-5115.

The Associated Press contributed to this report.