A 20-year-old black woman has told police she was set on fire by three men who wrote the initials KKK and a racial slur on her car.
Officers found Sharmeka Moffitt with burns on more than half of her body when they responded to her emergency call on Sunday night, said Louisiana State Police spokeswoman Lt Julie Lewis.
The FBI is investigating the attack as a possible hate crime, but no arrests have yet been made.
Lt Lewis said Ms Moffitt was in a critical condition in hospital and that some of her injuries were third-degree burns.
Ms Moffitt told police the men doused her in a flammable liquid and set her on fire at a park in Winnsboro, a town in Franklin Parish. She extinguished the fire using water from a spigot before a police officer arrived.
Officers found the letters KKK - an apparent reference to the white supremacist Ku Klux Klan - and a racial slur smeared in a paste-like substance on the bonnet of her car, Lt Lewis said.
On the emergency call, Ms Moffitt described her attackers as three men wearing white hoods or hats, Lt Lewis said. She later told a Winnsboro Police officer who responded to the call that the men were wearing white hoodies. She was unable to say what race her attackers were.
The officer found no suspects or vehicles at Civitan Park where the attack allegedly happened, and the park had no surveillance cameras, Lt Lewis said. She said the state crime lab was analysing several pieces of evidence.
Franklin sheriff Kevin Cobb called it "a horrific event" and said authorities would "follow the facts and seek justice".
Otis Chisley, the president of the local branch of the National Association for the Advancement of Coloured People (NAACP), a civil rights group, said he had been in touch with Ms Moffitt's distraught family. He said he was waiting for more facts to come to light before drawing any conclusions about what happened and that "everyone wants to move with caution."
Mr Chisley said racism and KKK activity remained a fact of life in the state.
"It's prevalent throughout Louisiana," he said. "It's hidden but it exists."