But the slain teen's mom, Sandy Smith, and South Carolina Highway Patrol officers didn't agree with Presnell's conclusion in the autopsy report.
In the Sept. 28, 2016, letter, obtained and published by FitsNews, Sandy wrote, "It has been apparent from the first week of this investigation that authorities are covering up critical evidence, and we no longer know who to trust."
MURDAUGH FALLOUT: LAWYERS FOR STEPHEN SMITH'S FAMILY ‘DON’T ACCEPT' ORIGINAL CAUSE OF DEATH
Buster Murdaugh, left, and Stephen Smith, right, a high school classmate that was killed in 2015. (Mark Sims for Fox News Digital/Facebook)
The grieving mother has been fighting for justice for her son since he was found dead nearly a decade ago on Sandy Run Road three miles from his car, which was out of gas.
The South Carolina Law Enforcement Agency (SLED) opened a homicide investigation into Smith's death in June 2021 – two weeks after Alex Murdaugh fatally shot his wife, Maggie, 52, and his 22-year-old son, Paul. He was convicted of the murders and sentenced to life in prison earlier this month.
ALEX MURDAUGH: TIMELINE OF ONCE-POWERFUL SOUTH CAROLINA LAWYER'S SPECTACULAR DOWNFALL
The family patriarch's downfall has ruined the Murdaugh name and undermined their dominance, which may make it easier for Sandy to finally get the answers she's been seeking.
Her lawyers, Eric Bland and Ronnie Richter, announced Wednesday in a press release that SLED Chief Mark Keel told them for the first time the agency does not believe Smith was killed in a vehicular accident but was the victim of an intentional homicide.
Dick Harpootlian and Eric Bland talk in Alex Murdaugh’s trial for murder at the Colleton County Courthouse on Thursday, February 9, 2023. (Andrew J. Whitaker/The Post and Courier/Pool)
In the 2016 letter, Sandy told the FBI that the Murdaughs were involved in her son's death and a subsequent coverup.
"The first call my family received after the murder was from authorities notifying us of Stephen's death," she wrote. "The second came very quickly the same morning from Solicitor Randolph Murdaugh."
The retired solicitor, who had served as the region's top prosecutor before retiring in 2005, is the late father of Alex Murdaugh.
He allegedly told her he was willing to work pro bono as a liaison between the family and investigators but soon stopped returning their calls, the letter says.
Within days of the killing, Stephen's twin sister was repeatedly approached by peers telling her that Alex Murdaugh's only surviving son, Buster, and his brother, Paul, who was later murdered, were responsible, Sandy wrote.
BUSTER MURDAUGH BLASTS ‘BASELESS RUMORS’ SURROUNDING 2015 DEATH OF CLASSMATE STEPHEN SMITH
A stranger told Stephen's brother, Chris, that he witnessed Buster "beat Stephen to death with a baseball bat" because he was gay.
Alex Murdaugh sentenced to life in prison after conviction in double murder trial during his sentencing at the Colleton County Courthouse in Walterboro on Friday, March 3, 2023 after he was found guilty of murdering his wife and son. ( Andrew J. Whitaker/The Post and Courier/Pool)
Sandy highlighted several anomalies at the crime scene – including the absence of tire marks or skid marks, which would be expected if her son was struck and slid across the asphalt. There was also no blood spatter.
"We desperately need your help," she told the FBI. "This investigation is being derailed."
Buster, 26, issued a statement Monday, through his attorney, Jim Griffin, denying any role in Stephen's death.
"These baseless rumors of my involvement in Stephen's death are false," he said. "My heart goes out to the Smith family."
He added that he is trying to cope with the death of his mother and brother and his father's convictions.