SAN DIEGO -- A body found Tuesday in a shallow grave on the shore of Lake Hodges is believed to be that of 17-year-old Poway High School senior Chelsea King who disappeared while jogging in Rancho Bernardo Community Park last Thursday, the San Diego County sheriff said Tuesdsay afternoon.

"Although positive identification has not been made, there is (a) strong likelihood that we have found Chelsea," San Diego County Sheriff William Gore announced in the late afternoon, shortly after breaking the news to the parents of the 17-year-old student.

A registered sex offender, John Albert Gardner III, was arrested Sunday in connection with Chelsea's disappearance and was booked into county jail on suspicion of rape and murder. Gardner is listed on the Megan's Law Web site, which indicates he lives in Lake Elsinore and committed lewd and lascivious acts on a child under 14 years old.

Gardner, 30, is scheduled to be arraigned Wednesday afternoon in a downtown San Diego courtroom.

A member of a law enforcement dive team found the body shortly after 1 p.m. on the south side of the lake, about 10 feet from the water's edge, Gore said.

"It's a heavily wooded area and not observable from the homes on the hillside or the park," he told reporters gathered at Rancho Bernardo Community Park, where Chelsea, an avid runner, disappeared Thursday afternoon.

The corpse was buried under a layer of earth and debris about a half-mile from where the girl had parked her car that day, according to Gore. He declined to say whether the body was clothed.

"I have just left the home of (Chelsea's parents) Brent and Kelly King, where I shared this information," the sheriff told news crews. "Obviously, they are devastated, and I ask that you respect their privacy during this extremely difficult time."

Word of the grim discovery began spreading in the late afternoon among the throngs of volunteers who had joined in the desperate effort to find Chelsea, a straight-A student and member of the San Diego Youth Symphony and her school's cross-country team.

Sheriff's officials abruptly postponed a planned 3 p.m. update on the case, then rescheduled the briefing for an hour later.

As the news broke, some of those who had been donating their energies to the search sobbed and hugged each other for support.

A candlelight vigil was held Tuesday night for Chelsea at St. Michael's Catholic Church on Pomerado Road in Poway.

"I want to thank you," Chelsea's father Brent King said. "Chelsea wants to thank you. Keep her spirit alive for us."

Tuesday morning, hundreds of good Samaritans joined law enforcement personnel in fanning out over fields, swamps and marshes around Lake Hodges for a sixth straight day in a determined effort to find the girl.

About 1,600 volunteers searched for Chelsea on Sunday, while about 1,000 went out Monday, according to figures posted on her family's Facebook page, which they created following her disappearance, in addition to the Web site findchelsea.com. In all, search organizers estimate about 6,000 volunteers helped looked for her, including about 1,300 Tuesday.

Since Chelsea's family reported her missing Thursday evening, a number of law enforcement agencies, including the FBI, pitched in to find her, scouring the Lake Hodges area by foot, air and boat, using sonar for underwater searching.

The sheriff has said physical evidence led them to Gardner, but declined to elaborate. Multiple sources close to the case told 10News Chelsea's underwear had been found near where the she disappeared.

Detectives also have linked Gardner to another attack late last year on a jogger in the same northern San Diego park where Chelsea was last seen, according to SDPD Capt. Jim Collins.

The morning of Dec. 27, a Colorado woman in the San Diego area on a family visit was running alone in the area when a man tackled her and demanded money. The 22-year-old victim, who managed to escape after elbowing the assailant in the face, left town before she could work with investigators on a sketch of the assailant, according to Collins.

Authorities say they also are investigating whether Gardner might be responsible for the February 2009 disappearance of Escondido High School freshman Amber Dubois, who was 14 when she vanished while walking to campus.

According to public records, Gardner was convicted of assaulting and molesting an adolescent girl in 2000. The victim, a 13-year-old neighbor, said he repeatedly punched her in the face and fondled her at his mother's townhouse, which is about a mile from Rancho Bernardo Community Park. The girl said she had gone to the residence after Gardner invited her in to watch a movie.

Gardner was sentenced to six years in prison and served five before he was released in September 2005. He was on probation until 2008. He registered as a sex offender using his grandmother's Lake Elsinore address but reportedly had been staying with his mother lately.

Dr. Matthew Carroll, a psychiatrist who interviewed Gardner following the 2000 assault, described him as "callous" and lacking remorse, making him "an extremely poor candidate for any sexual offender treatment," according to court records.

"It is my opinion that (Gardner) would be a continued danger to underage girls in the community," Carroll wrote as he recommended the maximum sentence of 30 years in prison.

A memorandum from the District Attorney's Office said prosecutors asked for a six-year sentence to resolve the case without a trial and to spare the victim from testifying.

While awaiting his first court appearance in the alleged murder of Chelsea, Gardner was being held in solitary confinement "so he can be observed and (authorities can) make sure he doesn't harm himself or nobody else harms him," the sheriff said.