Investigators are now 'cautiously optimistic' because they say they know who was with Jessica during the so-called 'mystery hour'

More than a year has gone by since the grisly murder of 19-year-old Jessica Chambers, but despite a thorough investigation that has combed a small Mississippi town, the killer or killers remain at large.

On December 6, 2014, Chambers was discovered badly burned but alive by first responders on Herron Road in Courtland, Mississippi, a town of just 500 people. Since then, District Attorney John Champion tells PEOPLE that investigators have interviewed 120 people, but nobody has been arrested in connection to her murder.

“If I knew [why the case was so hard to solve] I would be heading to Vegas,” Champion told PEOPLE in early 2015.

Still, Champion tells PEOPLE that his office continues to make progress. A reward of $54,000 has been offered to anyone for information that leads to an arrest in the case and more than 155,000 Facebook users have “liked” the Justice for Jessica page.

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.

Investigators have established an in-depth timeline for Jessica’s last day alive, and now say they know who she was with during the so-called mystery hour before she is known to have shown up at Herron Road.

“That hour holds the key to solving our case,” Champion tells PEOPLE, adding, “I’m cautiously optimistic.”

And earlier this month, authorities arrested 17 suspected area gang members on gun, drugs, and counterfeit charges.

“I don’t anticipate any of these [arrests] being directly tied to the Chambers case,” Champion tells PEOPLE. “But in their plea negotiations, all will be given the opportunity to help themselves and maybe that will bring us closer to the truth.”

A source at the scene reported 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.

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, ex-boyfriend Bryan Rudd tells PEOPLE.

“I think she was making friends with the wrong people, with drug dealers,” Rudd says.