A Georgia sheriff who recently closed the case of a 17-year-old student found dead on a rolled-up gym mat in 2013 announced Monday that he would pay $500,000 to anyone with information leading to an arrest or conviction of his own money offers .

Last month, Lowndes County Sheriff Ashley Paulk released a 16-page document in which he concluded that Kendrick Johnson’s death was an accident and no charges would be filed.

Johnson’s family said they believe the teenager was murdered and his death covered up.

“Following the release of my summary of the federal files on the Kendrick Johnson case, his parents called me a liar and continue to claim that Kendrick was murdered,” Paulk said in a press release Monday.

“Based on these testimonies, I am personally offering, through my own resources, a half-million dollar ($500,000.00) reward to anyone who provides information leading to the arrest and conviction of any person in the alleged murder of Kendrick Johnson of Lowndes High School.”

Johnson’s body was found on a wrestling mat in the high school gymnasium in Valdosta, Georgia on January 11, 2013. According to the report, his body was lying head down in a rolled-up mat positioned vertically. His feet were visible from the top of the mat, the report said.

State and local law enforcement officials ruled the death as accidental asphyxiation and said Johnson died after climbing onto the mat to retrieve his sneakers.

Kendrick Johnson’s body was found upside down on a rolled up wrestling mat at Lowndes High School in Valdosta, Georgia on January 11, 2013. Investigators decided his death was an accident.Courtesy of the Johnson family

The sheriff said Tuesday Johnson’s death, while tragic, was just a “strange accident.” Paulk said he and two detectives poured about 17 boxes of files over a year before releasing the report.

The half-million-dollar reward is “a last resort” for any information that the sheriff’s office may not be aware of, he said.

“I’m willing to raise those funds to see if anyone comes up with anything useful that may have been overlooked,” Paulk told NBC News on Tuesday.

“I don’t really feel like there’s anyone out there with knowledge and from what I saw – I’ve studied all the camera angles and everything else – it was a tragic accident. But if someone comes in with a different view, we’ll definitely listen to them.”

Paulk said he hasn’t received any useful tips since the reward was announced. The Johnson family and their attorney were not immediately available Tuesday.

“I’ve looked at everything I can see. If there’s anything else that anyone knows that they feel we haven’t looked at or studied, sit down with us,” the sheriff said.

The files, which Paulk released in his final report last month, included material from the Justice Department, the FBI, the Georgia Bureau of Investigation, the US Attorney’s Office for Central Georgia and several other law enforcement agencies.

It also included federal grand jury testimony from 58 people, as well as additional autopsies by a physician contracted by the Johnson family and the Department of Defense.