Investigators now know who the 19-year-old was with before she showed up at Herron Road, where she was found burned alive

Mississippi authorities investigating last year’s brutal murder of 19-year-old Jessica Chambers say they know who she was with during the so-called “mystery hour” before she showed up at Herron Road in Courtland, where she was found burned alive in her car last December 6.

Investigators have established an in-depth timeline for Jessica’s last day alive, but have been seeking more information about what she was doing between 6:30 until 7:32 p.m., when she arrived on Herron Road.

“That hour holds the key to solving our case,” District Attorney John Champion tells PEOPLE. Now that authorities believe they know who Jessica was with, Champion tells PEOPLE, “I’m cautiously optimistic.”

After a passing motorist called 911, first responders discovered Chambers clinging to life with wounds covering 98 percent of her body. A source at the scene tells PEOPLE that when first responders found Chambers barely clinging to life, she whispered one word: a man’s name, either, “Eric” or “Derrick.” Champion tells PEOPLE he has heard that detail, but he can’t confirm its validity.

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Though she graduated from high school with A’s and B’s and held down a job, Chambers was running with a rough crowd prior to her death, an ex-boyfriend tells PEOPLE.

“I think she was making friends with the wrong people, with drug dealers,” ex-boyfriend Bryan Rudd tells PEOPLE.

But he adds that “she did nothing that was bad to get herself burnt alive.”

For more on the mysterious case of Jessica Chambers, including a timeline of her last day and new information from police, pick up this week’s copy of PEOPLE on newsstands now.

“I write her, ‘I love you to the moon and back my sweet, precious angel. I miss you.’ ”