The review by the Colorado Bureau of Investigation was announced in September after Thompson's widow, Anita Thompson, contacted authorities with "new concerns and potential information regarding the investigation" into Thompson's death, the agency said in a news release.
In a statement included in the announcement, she thanked the bureau for its "kind and thorough work".
"This allows all of us who loved Hunter to move forward with a clean conscience," she said.
Thompson, who popularised a first-person form of gonzo journalism in articles and books such as Fear and Loathing in Las Vegas, died at his Aspen-area home in February 2005. He was 67.
His remains were famously cremated and fired from a cannon at his request during a private ceremony in Colorado. Among the celebrities who attended was actor Johnny Depp, who played the lead role in the 1998 film adaptation of Fear and Loathing.
The sheriff for the Aspen area, Michael Buglione, said he believed the original investigation into Thompson's death was conducted properly but investigators recognised the importance of an independent review.
"CBI's conclusions reaffirm the original findings and, we hope, provide reassurance and clarity," he said.
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