Authorities this week charged Quinton Tellis, 27, with capital murder after tracing phone records which led them to believe he was the last person with Chambers, 19, on Dec. 6, 2014. That evening she was found burning alive in her car in the small town of Courtland, Mississippi, and later died from her wounds.
Tellis currently is being held in a Louisiana jail, charged with unlawful use of a credit card that belonged to a murdered Taiwanese exchange student. Although he's not been charged in that August stabbing death, the district attorney in the Chambers case, John Champion, says Louisiana authorities have "a really good circumstantial evidence case on that murder" as well.
Speaking to PEOPLE by phone, Jessica's stepmother Debbie Chambers says: "[I think] this isn't the first time. I believe he's killed before. This is my opinion."
Champion told PEOPLE that Tellis, who grew up in the same neighborhood and attended the same high school as Jessica, was a "habitual offender" who served time for burglary and fleeing arrest.
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"I really don't think this was a girlfriend-boyfriend. I don't think so. I just think maybe they was just friends, acquaintances, or something like that."
"I had never heard of this guy before," she adds. "None of us heard of him."
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Chambers' Stepmom Says 'Her Daddy's Not the Same Person Anymore'She says the long months of waiting for an arrest have been hard on the family, especially Jessica's father Ben, who told PEOPLE he thought Tellis never should've been out on the street.
Says Debbie Chambers: "Her daddy's not the same person anymore. He's not the same man. He's different now."
"We don't go anywhere anymore, we don't do anything. We always were doing family functions and all that. We may meet now for Easter, have an Easter meal, or Christmas, have a Christmas meal, but we don't do like we used to do.
"We all used to gang up and go to the lake and whatever, go to the zoo, but we just don't do it now. He just doesn't want to."
Although the family had been alerted that the arrest of Tellis was imminent, she says there is still much they don't know.
"I was relieved, but then I was shocked because you know, here he is, because he got away with it, and he was able to go and kill again," she says.
"There's a lot of stuff they're still not telling us, because they say it's going to interfere with their case and the trial. We still are in the dark about a lot."
Tellis has not yet entered a plea to the murder charge.
Reporting by ALEXANDRA ROCKEY FLEMING