A convicted sex offender charged with murdering Chelsea King in Southern California is under investigation in last year's disappearance of a 14-year-old girl and the attempted kidnapping of another teenager, authorities said.

John Robert Gardner III remained jailed Thursday in San Diego County, a day after he pleaded not guilty to murdering the 17-year-old King.

The sheriff believes a body found Tuesday in a shallow grave on the shore of Lake Hodges is that of King, but the medical examiner's office had not yet confirmed the identity.

Gardner, once described by a psychiatrist as a threat to underage girls, was under scrutiny in two other cases.

A 16-year-old girl reported that she ran away after a man asked her for directions then tried to force her into a car at gunpoint on Oct. 28 in Lake Elsinore, about 75 miles from the site where King disappeared. the Riverside County Sheriff's Department said.

The suspect was described as a man 30 to 35 years old with a squarish jaw, brown eyes and a blond crew cut. A sketch appears similar to Gardner, who is 30 and has a closely shaven scalp.

"There are similarities," Capt. Joseph Cleary told the Riverside Press-Enterprise. "If there is a connection, we will track it down."

Investigators in the case checked out local sex offenders but not Gardner because he was registered as living in San Diego County, Sgt. Patrick Chavez said.

"He had not come onto our radar until January" when he notified authorities that he was now living in unincorporated Lakeland Village near Lake Elsinore, Chavez said.

Gardner lived last year in the San Diego County town of Escondido, where 14-year-old Amber Dubois vanished while walking to Escondido High School on Feb. 13, 2009.

Gardner lived about two miles from the school. Authorities said they are now investigating him in connection with the girl's disappearance.

Amber's father, Maurice Dubois, was one of thousands of volunteers who searched for King after the Poway girl failed to return from a run at a San Diego wilderness park on Thursday. Dubois also attended Gardner's court hearing.

Dubois said he feels there is a "huge possibility" that Gardner is linked to his daughter's disappearance.

"In our hearts we don't want to accept that," he said.

Dubois said it was difficult to see Gardner in court.

"My frustrations are immense," he said. "We've had a long, long year."

Gardner also pleaded not guilty to assault with intent to commit rape in connection with a Dec. 27 attack on a 22-year-old jogger at the same park.

Gardner pleaded guilty in 2000 to molesting a 13-year-old neighbor girl. He served five years of a six-year prison term. A court-appointed psychiatrist who interviewed Gardner had recommended he receive a stiffer sentence.

Dr. Matthew Carroll called Gardner a poor candidate for therapy and a "continued danger to underage girls in the community."