(CNN) -- Forensic evidence found on property belonging to a missing California teenager led authorities to a man suspected of kidnapping and killing her, authorities said Tuesday.
Sierra LaMar, 15, of Morgan Hill has been missing since March 16, when she left her mother's home to walk to her school bus stop.
Monday night, police arrested Antolin Garcia-Torres, 21, also of Morgan Hill. He faces charges of murder and kidnapping in LaMar's disappearance.
Santa Clara County Sheriff Laurie Smith told reporters Tuesday that investigators believe LaMar is dead, although they have not located her body.
"We continue to pray until she's found," LaMar's mother, Marlene, said Tuesday. "Our search still is not going to end."
She implored Garcia-Torres to "come forward and say where she is."
Smith said Garcia-Torres has not been forthcoming with authorities. "We have gained some value from our interviews with him, but nothing substantive," she said.
LaMar's purse and phone were found at different locations on the side of the road within two miles of the house. The purse had clothes neatly folded inside.
Forensic evidence was found "in multiple places" on LaMar's possessions, Smith said. She would not divulge the nature of the evidence. Physical evidence also links LaMar to Garcia-Torres' vehicle, which was seized on April 7.
Garcia-Torres was identified March 28 after forensic evidence was run through a law enforcement database, Smith said. Garcia-Torres was in the database because he had a prior felony arrest for an assault, although the case was not prosecuted, she said.
Since then, authorities have had him until 24-hour surveillance, she said. "We were hoping that he would lead us to where Sierra was."
The motive for the abduction is not known, she said, but "it's my belief this was purely random. It was an absolute stranger abduction."
LaMar was "a very social girl" who was constantly in touch with her friends by telephone and through social media, she said. Since she was reported missing, she has contacted no one. In addition, some property found was items "she would have taken with her, including medication," the sheriff said.
Investigators believe Garcia-Torres acted alone, she said.
"These are very, very difficult cases, to prosecute a homicide when you have not found the victim, but it has been done and I think we have adequate facts, strong facts" to show the teen is dead, Smith said.
Authorities have not recovered a weapon and don't know how long LaMar might have been kept alive or how she died, she said.
Forensic evidence has also linked Garcia-Torres to one of three unsolved assaults in Morgan Hill in March 2009, Smith said. In one assault, a stun gun was used on a victim who managed to escape; another victim had her own knife used against her; and in a third, a man tried to get into a woman's car but was unsuccessful when she locked it.
Smith said she could not remember which of those three Garcia-Torres was linked to.
In April, Marlene LaMar, described her daughter as a focused student who was a cheerleader.
"She was passionate," Marlene LaMar told HLN's Nancy Grace. "She did dance, competitive dancing. And she also enjoyed cheerleading. She was driven. You know, she set the bar kind of high for herself as far as, you know, trying her best."
Morgan Hill is about 22 miles southeast of San Jose.
"We still need your support," the girl's father, Steve LaMar, told reporters Tuesday. "We need to bring Sierra home. ... We want justice served. That's what we're looking for."
CNN's Greg Morrison contributed to this report.