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 Mr. Carpenter, a businessman 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 Mr. 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.