A 30-year-old sex offender was charged Wednesday with murdering a 17-year-old San Diego County girl in a case that could end in the death penalty.

Prosecutors say John Albert Gardner III also faces a special allegation of rape or attempted rape involving the teen, Chelsea King, who vanished last week after heading off for a run in a park. Police say a body found Tuesday in a shallow grave is likely that of King.

Gardner also was charged with assault with intent to commit rape in an unrelated Dec. 27 incident.

An attorney entered pleas of not guilty on Gardner's behalf during his brief court appearance Wednesday.

The district attorney has the option to pursue the death penalty, but that determination has not been made yet.

Click here for a video of the suspect in court.

A huge crowd had gathered outside the courtroom during the arraignment. Inside the courtroom, tissues were passed out to Chelsea King's family and friends, who were wearing ribbons in her honor.

Gardner looked down at his handcuffed hands throughout the arraignment and said nothing. His next court appearance will be March 18.

Gardner, a Lake Elsinore resident, was arrested Sunday. Police said a spray-painted message found on the his mother's garage read, "Chelseas (sic) blood is on you. Move out." Police did not know who painted it.

Click here to read more about the parent's home being vandalized.

Gardner lived at the home in 2000 when he molested a 13-year-old neighbor. The home is down the street from an elementary school and near the park where King was last seen Thursday wearing running clothes.

A piece of paper taped to the front door told visitors to leave.

As prosecutors prepared their case against Gardner, the county medical examiner's office worked to confirm King's body had been pulled Tuesday from a shallow grave on the south shore of Lake Hodges.

An investigator was meeting with King's parents at their Poway home, said Lenore Aldridge, a medical investigator.

It was not clear if fingerprints, dental records or other methods of identification were being used.

County Sheriff William Gore said the body was likely King.

Thousands of people joined the search after the teen failed to return from a run near the lake on Thursday.

Mourners held a candlelight vigil Tuesday night for the popular straight-A student at Poway High School.

"What bothers me most is the kids don't feel safe anymore," Traci Barker-Ball, King's teacher for three years, said between hugs of grieving students. "Their sense of security has been taken away from them."

Brent King, Chelsea's father, thanked supporters on the lawn of St. Michael's Church in Poway, northeast of San Diego. Some people in the audience fought to control their sobs as he spoke.

"One of the nicknames that I've always called my daughter is my Angel. She's my angel forever," he said. "I want to thank you. Chelsea wants to thank you."

Someone shouted, "We love you," and Brent King replied, "We love all of you."

Gardner was arrested after police said a piece of physical evidence tied him to King. No further details were provided.

Gardner pleaded guilty in May 2000 to molesting the 13-year-old female neighbor and served five years of a six-year prison term. Prosecutors said he lured the victim to his home with an offer to watch "Patch Adams," a 1998 movie starring Robin Williams.

The girl was beaten before escaping and running to a neighbor.

Gardner "never expressed one scintilla of remorse for his attack upon the victim" despite overwhelming evidence, prosecutors wrote in a sentencing memo.

He had faced a maximum of nearly 11 years in prison under terms of a plea agreement, but prosecutors urged six years.

Dr. Matthew Carroll, a psychiatrist who interviewed Gardner, wanted a stiffer punishment, saying in court documents that he was a "continued danger to underage girls in the community."

Gardner was on parole for three years, until September 2008, state records show.

San Diego police said Gardner also was linked to an assault on a 22-year-old Colorado woman who managed to fend off her attacker on Dec. 27 in Rancho Bernardo Community Park on the northern edge of San Diego, where King's 1994 BMW was found with her belongings inside.

San Diego police Capt. Jim Collins declined to describe the evidence connecting Gardner to the December assault but said a swab taken from the victim's elbow did not match Gardner's DNA.

Fox News's Adam Housley and The Associated Press contributed to this report.