(CNN) -- Authorities believe that a registered sex offender who this month admitted killing two California teenagers attempted to follow an 11-year-old girl walking home from school the day before one of the teens disappeared, according to documents filed in the case.
John Albert Gardner, 31, pleaded guilty April 16 to killing Chelsea King, 17, and Amber Dubois, 14. He also pleaded guilty to a charge of assault with intent to commit rape in a December incident involving a third woman.
King was last seen leaving Poway High School in suburban San Diego, California, on February 25. Her car, with her cell phone inside, was found at Rancho Bernardo Community Park. King was known to run on the park's trails. Her disappearance triggered a massive search that ended a few days later, when King's remains were found in the park.
Dubois disappeared in February 2009 while walking to school in Escondido, California. She was considered a missing person for more than a year until her remains were found in March. Prosecutors said Gardner led police to Dubois' body after being assured that it would not be used against him in court.
Gardner avoided the death penalty with his guilty plea but will be sentenced June 1 to two consecutive life terms without the possibility of parole, under terms of the plea deal with prosecutors.
On February 24, a day before King went missing, an 11-year-old girl reported that she was walking home from school when she heard a vehicle slowly approaching her from behind, according to an affidavit requesting a police search of Gardner's home, posted on the website of CNN affiliate KTLA-TV. The documents were unsealed Monday, KTLA reported.
The girl told police the vehicle drove past her and then parked about 30 feet in front of her, the documents said. The driver stayed in the vehicle, the girl said, and did not attempt to speak to her. But a woman pulled up next to the girl and told her the person in the vehicle "seemed very suspicious" and offered to follow her home, according to the affidavit.
The man, who was in a black car, then made a U-turn and left, and the woman followed the girl the remaining two blocks to her house, the documents said. The girl's mother called police after her daughter told her what happened.
The girl's mother told authorities that after seeing a photo of Gardner following his arrest in the King case, her daughter said Gardner was "the guy in the car," the affidavit said. Gardner's girlfriend drives a black 2002 Nissan Sentra, according to the documents.
The documents also detail the December assault on the third woman near where King's car was found at the park. She told authorities she was on a hiking trail and a man walked past her, the affidavit said. She told him, "Good morning," and he returned the greeting, but then tackled her from the side as he passed her, knocking her to the ground on her back.
The man pinned her to the ground, and she screamed while struggling with him, the affidavit said. He told her to "shut up," and she said, "You're going to have to kill me."
"That can be arranged," he responded. The man then demanded money from her, the affidavit said.
She was able to sit up and used her elbow to strike him in the nose, telling police she felt "the crunch of cartilage." The man let go of her and grabbed his nose, she said, and she ran.
A woman also told police that she saw Gardner on the jogging trails near the park on the day King disappeared, the affidavit said. She said he was sitting and drinking a beer and had several empty beer cans around him. He warned her about a rattlesnake that was nearby, and they spoke briefly.
The woman said she ran past the man and then talked to him some more on her return trip. The man was wearing blue jeans and a Hard Rock Cafe T-shirt, she said, according to the affidavit.
A second woman also recalled seeing a man in the Hard Rock Cafe shirt on a trail that day about 3 p.m., about an hour and a half after the first woman said she saw him, the affidavit said. The same woman believes she saw King on the trail as well. Both women told police they were 100 percent sure the man they saw was Gardner, the affidavit said.
The area where the second woman recalled seeing Gardner was "300 yards from where the panties of Chelsea King were found and about a mile from where the body of Chelsea King was found," the affidavit said.
At the time of his arrest, Gardner was a registered sex offender. In 2000 he was convicted of two counts of lewd and lascivious acts with a child under 14 and a count of false imprisonment, according to a probation officer's report. The victim in that case was a 13-year-old girl, who was molested and beaten when she tried to resist, the report said.
"The defendant ... manifests marked predatory traits and is not seen as a suitable candidate for sex offender treatment insofar as he refuses to accept any responsibility for what he has done," according to the 2000 report.
Gardner knew the girl and was a former neighbor of hers, the report said. "The fact that he would try to force himself on her sexually is indefensible but then to beat her as he did out of frustration ... represents a qualitative leap to extremely serious criminal behavior."
The report recommended that Gardner be sentenced to six years in prison. KTLA reported that he served five years before being paroled.