More than six decades after the murder of President John F. Kennedy, never-before-seen footage of the assassination’s immediate aftermath has come to light.

A minute-long, 8mm color film — the existence of which was previously unknown — will soon be sold through RR Auction. Online bidding is now underway, and the live auction is scheduled for Sept. 28.

The unearthed footage, in two parts, shows the president’s motorcade driving in downtown Dallas and then speeding toward Parkland Hospital as Clint Hill, Jackie Kennedy’s Secret Service agent, shields both her and the president.

After the first shot rang out on Nov. 22, 1963, Agent Hill leapt onto the back of the presidential limousine and placed his body spread-eagle over the back seat.

Now 92, Hill tells PEOPLE that the discovery of the film is “an emotional thing for me because I know what has happened. I know what I’m seeing in that film is a dead president.”

The footage was shot by Dale Carpenter Sr., a local Dallas truck driver who filmed from Stemmons Freeway. Carpenter’s grandson, James Gates, tells PEOPLE that the film landed in his possession when his mother handed down family mementos sometime after his grandfather’s death from Parkinson’s disease.

“She gave me several reels of family films and audio recordings, a projector and the reel-to-reel audio recorder,” Gates says. “The film was stored in a plastic Dean Foods milk crate and had been in my closet for several years before I decided to put it in a fire safe.”

Bobby Livingston, executive vice president at RR Auction, says, “Unlike any other footage known to exist, this film captures a segment immediately following the Zapruder film, providing a fresh perspective on one of the most analyzed moments in history.”

“Once you see it,” Livingston adds, “you’ll never forget Clint Hill’s heroic efforts to protect Mrs. Kennedy, racing at 80 miles an hour in a desperate attempt to save the president’s life as they sped to Parkland.”

The Zapruder film, taken by clothing manufacturer Abraham Zapruder, as the presidential motorcade passed through Dealey Plaza, is considered the most complete — and graphic — footage of the assassination, whereas the new footage depicts the aftermath as the presidential motorcade races to the hospital and Mrs. Kennedy leans over her wounded husband.

“It brings it back so vividly,” says Agent Hill’s wife, Lisa McCubbin Hill, with whom he wrote Mrs. Kennedy and Meand three additional bestsellers.

“It really brings the moment to life,” she continues. “When you see the president’s limousine, you see Clint on the back of the car. To see the speed of the car and how precarious that was — Clint is still trying to protect the president and Mrs. Kennedy from whatever else might be coming. One slip and he would have gone flying off and he would have been killed.”

There are two segments to Carpenter’s film. PEOPLE obtained the first segment, which can be seen below, that shows the presidential motorcade before it arrived at Dealey Plaza.

As Hill explains, Carpenter “initially missed seeing the president in his open top car, but he captured the rest of the motorcade. You see the back of the follow up car and then he made his way to Stemmons Freeway because the president was on his way to the Dallas Trade Mart to give a speech.

The second segment of the film shows the motorcade’s dramatic race to the hospital after President Kennedy was shot.

Not knowing at that point that Kennedy had been injured, Carpenter stood on the shoulder of the Stemmons Freeway hoping to catch a glimpse of the president. That’s when he captured the footage showing the first lady wearing her now-iconic bright pink suit, draped over the wounded president.

Gates decided to track Agent Hill down around 2012 to let him and his wife know that his grandfather had been holding onto private footage from the assassination.

“About 10 years or so ago, I started to research the JFK assassination, knowing I had something possibly of some historical value,” Gates says. “I found Clint and Lisa, but it wouldn’t be until another 10 years when I made the decision to properly preserve and archive the film.”

The Carpenter film will now be included in an upcoming documentary about Hill called Agent Number 9: The Clint Hill Story, which is set for release in 2025.

As remarkable as the film is, Hill says matter-of-factly, “It doesn’t change anything for me.” Nor does it bring the events of that horrible day back any more than usual.

“I’m back there all the time anyways,” he says.