(CNN) -- A California woman missing for nearly a year and believed to be dead may still be alive and living in the Las Vegas, Nevada, area, detectives from Los Angeles said Thursday.
Investigators have spent about six weeks checking out a high-school classmate's report that he saw Mitrice Richardson at a hotel casino, and are "unable to show that it is not a good lead," Los Angeles police Capt. Kevin McClure said. And detectives have talked to several witnesses since then who believe seen her in the area, he said.
"It is not a verified sighting of her here, but we feel good enough to come up here and spend a good deal of time to get the information out to the community that we believe she might be here," McClure told reporters at a news conference in Las Vegas.
But Richardson's mother, who is suing the Los Angeles County Sheriff's department over her daughter's disappearance, cast doubt on the report. Latice Sutton told HLN's "Issues with Jane Velez-Mitchell" that the man who reported the sighting hasn't seen her daughter for a decade.
Richardson, who would be 25 now, is a former beauty pageant contestant who was last seen leaving a Los Angeles County sheriff's station in Malibu in the early morning hours of September 17, 2009. She had been arrested the previous evening at an upscale restaurant for allegedly not paying for her meal, and patrons at the restaurant said Mitrice exhibited strange behavior.
Her family has said the college honors graduate suffered from mental health issues and should have been kept at the sheriff's station until a relative arrived to pick her up.
Investigators are now asking the public in and around Las Vegas to report any possible sightings, and urged Richardson herself to come forward.
"She's not in any type of trouble," said Capt. Dave Smith, of the Los Angeles County Sheriff's Department. "She's not subject to any type of arrest We would like her to approach her family, approach us, and please let us know that she's OK."
A high-school classmate now living in Las Vegas reported seeing Richardson at the Rio Hotel and Casino in early June, sparking the new effort by detectives to check out his report.
"He walked up and said, 'Hi, Matrice," Smith said. "She basically looked at him with kind of a shocked look and left the location."
The casino's security cameras didn't have video of the incident that was of a high enough quality. But since then, investigators have talked to more than 70 people who say they have seen her, said William McSweeney, chief of the Los Angeles County detective division.
Those accounts were "based on a photograph and some passage of time, in most instances," McSweeney said. "But that volume causes us to beleive we're on the right track."
But Sutton told HLN on Thursday, "I do not believe it was her." She said hundreds of similar reports in Los Angeles have failed to pan out, while the man who reported seeing her in the casino hadn't seen her daughter since high school. And she questioned why police considered that report believable after dismissing an earlier report by Michael Richardson, the missing woman's father, that he had spotted his daughter in Las Vegas.
"Someone who hasn't seen Mitrice in 10 years, they consider him now more credible than the biological father?
Sutton asked. "I do not believe this is Mitrice. Mitrice was last seen at the sheriff's department."
Last month, Sutton sued Los Angeles County and several sheriff's officials for wrongful death and negligence in her daughter's disappearance, according to court documents. Sutton argued that the sheriff's department failure to administer psychiatric or medical evaluations and the fact that Richardson was released "alone in an unfamiliar area without money, a cellular phone or means of transportation amounts to negligence." The lawsuit seeks unspecified monetary damages.
Steve Whitmore, a spokesman for the sheriff's department, told CNN in September that the decision to release Richardson was made because "she was not intoxicated, she didn't exhibit any mental issues, so when we were done running her fingerprints and criminal history, then we are obligated by law to release her from custody."
He also has said that a female jailer "offered to her to stay the night. She could have stayed, but she wanted to leave."
Police have not discounted the possibility that Richardson may not want to be found, but her family believes that would be "out of her character" McClure said.
"We could find that out, but we'll never know that until we actually talk to Mitrice," he said.
CNN's Gabriel Falcon contributed to this report.