NEW YORK, NY.- Nearly 61 years ago, Dale Carpenter Sr. showed up on Lemmon Avenue in Dallas, hoping to film John F. Kennedy as his motorcade passed. But the president’s car had already gone by, and he recorded only some of the procession, including the back of a car carrying Lyndon Johnson and the side of the White House press bus.

So Carpenter, a businessperson from Texas, rushed to Stemmons Freeway, several miles farther along the motorcade route, to try again.

There, just moments after Kennedy had been shot, he captured an urgent and chaotic scene. The president’s speeding convertible. A Secret Service agent in a dark suit sprawled on the back. Jacqueline Kennedy, in her pink Chanel outfit, little more than a blur.

Kennedy himself could not be glimpsed. He had collapsed and was close to death.

For decades Carpenter’s 8-millimeter snippets of what transpired in Dallas on Nov. 22, 1963, have been a family heirloom. When he died in 1991 at 77, the reel, which included footage of his twin boys’ birthday party, passed to his wife, Mabel, then to a daughter, Diana, and finally to a grandson, James Gates.

Later this month, the Kennedy footage is to be put up for sale in Boston by RR Auction, the latest in a line of assassination-related images to surface publicly after decades in comparative obscurity. The auction house says it is the only known film of the president’s car on the freeway as it sped from Dealey Plaza, the site of the shooting, to Parkland Memorial Hospital, where Kennedy was pronounced dead at 1 p.m.

Footage shot by Abraham Zapruder, a bystander, has long provided disturbing images of the assassination itself, one of the most traumatic and closely examined events in American history. Carpenter’s film shows what happened before and just after the Zapruder film was shot. The first section is a prosaic scene of the president’s motorcade; the second, a race for help imbued with all the uncertainty that filled the moments after the gunshots.

Though Carpenter’s film, just over a minute long, contains nothing likely to affect the debate over Kennedy’s death, several experts said it is still an important addition to the mosaic of images that recorded that day in Dallas.

“It almost is a little bit of a coda or addendum to the Zapruder film,” said Gerald Posner, the author of “Case Closed: Lee Harvey Oswald and the Assassination of J.F.K.”

Posner said Carpenter’s film “captures the heartbreak of what just happened in Dealey Plaza and what we know we’re about to find out soon at Parkland hospital.”

The auction house said that bidding on Sept. 28 will begin at $5,000, and that it estimates the film has a value of more than $100,000.

Only a handful of people outside Carpenter’s family have seen the footage, according to the auction house. Among them are Posner; Mark S. Zaid, a lawyer who has written about and lectured on the assassination; and Clint Hill, the Secret Service agent who sprinted from a car behind Kennedy’s and climbed onto the trunk of the president’s Lincoln Continental after the first shot rang out. All three said they believe the footage is authentic.

Hill said that in the moments filmed by Carpenter on the freeway, the president was lying across the back seat with his head in his wife’s lap. Hill is shown in a protective position with one foot inside the car and another outside as it races to the hospital, a trip that took several minutes.

“It felt like a lifetime,” Hill said in a telephone interview. “Every second counted.”

Stephen Fagin, the curator of the Sixth Floor Museum at Dealey Plaza, in Dallas, said the images of that day, created by journalists and ordinary citizens, are “the window through which we understand the moment of the assassination and the aftermath.” The museum has created a map of the motorcade route that features images by professional photographers and by the many spectators who carried still and movie cameras.

New images have continued to emerge over the decades. For instance, Fagin said a man named Jay Skaggs walked into the museum in 2002 with photographs he had taken in 1963, including the only known color ones of Lee Harvey Oswald’s Mannlicher-Carcano rifle being removed from the Texas School Book Depository building. Until then, those pictures had been seen only by Skaggs’ family.

“I view these images and films like puzzle pieces coming together to form this tapestry of memory of Nov. 22,” Fagin said. “A visual record of what it was like to live that experience.”

A concrete company executive, Carpenter lived in Irving, Texas, about 12 miles northwest of Dallas. He was not known to have a particular affinity for Kennedy, two of his children said, but was drawn to the pomp of the presidential visit and brought along the camera that he often used to film family events. One of his sons, David Carpenter, 63, whose birthday celebration was on the same reel as the Kennedy footage, said: “Daddy was always behind that camera.”

Dale Carpenter kept the film he had shot in a round metal canister labeled “JFK Assassination.” Screenings were rare, perhaps, as David Carpenter suggested, because the footage showed “something that was dramatic and terrible.” Family members said they were not sure whether their father ever spoke with authorities about the film.

Gates said he came into possession of the film in 2009 or 2010, when his mother handed him a milk crate that included some 30 film reels that had belonged to her father. He said he had been somewhat disappointed when he first viewed the footage because it did not show Kennedy. Still, he said he was struck by Hill’s bravery.

In 2012, Gates corresponded with Lisa McCubbin, who wrote a book with Hill about his time protecting Jacqueline Kennedy. Soon afterward he sent her and Hill a copy of the film.

New interest in the footage surfaced recently, when the makers of a yet-to-be-released documentary about Hill’s life, “Agent Number 9,” obtained the rights to include Carpenter’s footage in their film. At the same time, McCubbin, who had recently married Hill, put Gates in touch with RR Auction.

Leaders of the auction house, which has sold other Kennedy- and assassination-related items, like Oswald’s wedding ring, found the footage to be powerful.

“There’s this iconic American sequence of events that we’ve seen over and over,” said Bobby Livingston, the company’s executive vice president. “And all of a sudden, 60 years later, you get another view of it, another perspective.”

Josiah Thompson, the author of “Six Seconds in Dallas,” has called the film footage and photographs from people who were in Dealey Plaza that day “a great source of evidence.” He said, though, that he did not think the Carpenter footage had “particular significance,” mainly because it will not resolve lingering questions about the assassination.

It is not clear if Carpenter knew the president had been shot when he recorded the images along the freeway. He certainly knew what had happened by the time he returned to Irving.

His daughter, Deborah Thornton, who was 12 in 1963, recalled her father arriving home that day looking “very solemn.”

She said she remembered him telling her mother: “Dallas has changed forever.”

This article originally appeared in The New York Times.