Last words could lead to her killer

• Jessica Chambers' words to firefighters have "certainly given us a lead," prosecutor says


• DA: Investigators believe she received call, her phone will be "the key to everything"


• Emergency responders found her badly burned on a rural road in Panola County, Mississippi


• NEW: Sister describes Chambers as outgoing, funny, happy all the time

(CNN) -- Mystery surrounds the fiery weekend death of Jessica Chambers, but the Mississippi teen may have given firefighters a clue in her killing, a prosecutor said Wednesday.

The volunteer fire department that responded to her car blaze Saturday night got there quickly because it was at another call a couple of miles away, said John Champion, district attorney for Mississippi's 17th Circuit.

When firefighters arrived at the scene near Courtland, Mississippi, Chambers approached one of them and spoke. She was not on fire, as has been widely reported, the prosecutor said.

Champion didn't divulge what she said but told CNN, "It has certainly given us a lead we're following up on."

Chambers, 19, was loved by everyone who knew her, older sister Amanda Prince told CNN's Don Lemon.

"She was crazy. Very athletic. Outgoing. She was just ... Jessica," Prince said. "She was happy all the time. She made everybody laugh."

Prince said her sister had talked about becoming a nurse and also wanted to become an author.

Investigators looking at cell phone

Mississippi teen burned to death

Burned teen's dad: She was unrecognizable

Though authorities are releasing sparse details of the crime and investigation, they have determined an accelerant was used in the blaze, but it's not clear whether the accelerant was poured on the car or Chambers herself, Champion said.

The state fire marshal is processing the evidence to determine what type of accelerant, and it's possible the evidence will need to be sent to the Bureau of Alcohol, Tobacco, Firearms and Explosives in Atlanta, he said.

Prince said she was told the attacker had gotten into the car and set Chambers on fire inside the vehicle.

Peter Odom, an attorney who prosecuted some arson cases, told HLN's "Nancy Grace" that investigators are looking for something in particular when they interview people involved in the case.

"They're looking for someone with singed eyebrows and maybe burned hands. Because the burner often gets burned," he said.

Investigators believe her phone will be "the key to everything," the prosecutor said. She received a call before she left her house, he said.

An autopsy was performed but results have not been made public.

Sister: She was supposed to come home

Despite reports she was going to a party, her appearance -- her hair was in a bun and she was wearing camouflage pajama pants -- suggests that wasn't the case, Champion said. Police have also spoken to partygoers, who don't recall seeing her there, he said.

Prince said her sister called their mother from a gas station and said she would be home right after she cleaned her car.

The prosecutor also said investigators found no evidence in a store surveillance video that appears to show Chambers prepaying for gas. She walks to the store's front door when something or someone catches her attention.

She waves and walks off camera briefly, comes back into the camera's view, enters the store where three men chat by the doorway and spends about a minute at the counter before going back outside and pumping gas.

The business, which Champion characterized as a rural mom-and-pop store, is about 2 miles from Chambers' home. She purchased something to drink there, the prosecutor said.

The clerk who helped her said nothing seemed out of sorts.

"She seemed normal," Ali Alsanai told CNN affiliate WREG. "She didn't seem like something was going wrong, you know? She just seemed normal. She just pumped some gas, we had a talk, and she left."

A Facebook page titled Justice for Jessica has drawn more than 75,000 likes and that number was climbing rapidly Wednesday. The page states it was created "to keep Jessica's name alive & out there."

"They have ripped everything I have," Jessica's mother, Lisa Chambers, told CNN affiliate WMC.

CNN's Steve Almasy, Dave Alsup and John Murgatroyd contributed to this report