James R. Leavelle, the big man in the white Stetson who epitomized the horrors of the assassination of President John F. Kennedy in one of the most famous photographs of all time — the killing of Lee Harvey Oswald by Jack Ruby — died Thursday at a hospital in Denver. He was 99.

His death was confirmed by his daughter, Karla Leavelle.

James Leavelle, a veteran Dallas homicide detective who had survived the Japanese attack on Pearl Harbor in 1941, was handcuffed to Oswald and was leading him through a police station basement on Nov. 24, 1963, when Ruby, a nightclub owner, stepped out of the crowd and pumped a fatal bullet into the prisoner. The shooting, with Oswald’s pained grimace and Leavelle’s stricken glower, was chillingly captured by Robert H. Jackson of The Dallas Times Herald in an iconic photograph that won the Pulitzer Prize the following year.

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Moments earlier, he and Oswald had had an eerie exchange, Leavelle often later recounted. “Lee,” he recalled saying, “if anybody shoots at you, I hope they are as good a shot as you.”

To which, he said, Oswald replied: “You’re being melodramatic.”

At the time, two days after Kennedy had been gunned down in a motorcade through downtown Dallas, Oswald was a suspect in the killing of a Dallas police officer, J.D. Tippit, and had yet to be conclusively tied to the assassination. But after Leavelle asked him whether he had shot the police officer, Oswald aroused the detective’s suspicions by insisting, “I didn’t shoot anybody,” as if, Leavelle later recounted, there had been another shooting as well.

In the decades that followed, Leavelle was in constant demand as a speaker, invariably asked to recall the fateful moment. “I saw him, he was standing in the middle of the driveway,” he said of Ruby in an interview with The New York Times in 2006.

“He had a pistol by his side, I saw out of the corner of my eye,” Leavelle continued. “I jerked back on Oswald to get him behind me. I had my hand through his belt. All I succeeded in doing, I turned him so instead of dead center the bullet hit 4 inches to the left of his navel and 2 inches above.”

Another detective, L.C. Graves, on Oswald’s other side, grabbed Ruby’s pistol around the cylinder, preventing another shot, Leavelle recalled. “I could see Ruby’s fingers flexing on the trigger, trying to fire,” he said. He knocked Oswald to the floor, removed the handcuffs and got him loaded into an ambulance. “I tried to take his pulse but I never could detect any pulse,” Leavelle said. He remembered hearing a groan and sigh in the ambulance, which he said he later took as the moment of Oswald’s expiration, although he was pronounced dead at Parkland Hospital, where Kennedy had been pronounced dead two days earlier.

Leavelle joined the Dallas Police Department in 1950, but his life had hardly lacked drama before then. The son of farm parents, James Robert Leavelle was born Aug. 23, 1920, and grew up in northeast Texas near Texarkana. He joined the Navy out of high school in 1939 and was stationed at Pearl Harbor. He was on a destroyer tender that carried supplies to other ships when the Japanese bombed the fleet about a mile away on Dec. 7, 1941. He was unhurt in the attack, but while at sea in the Pacific during a severe storm in 1942, he fell off a ship’s ladder and had to be evacuated to a naval hospital in California.

There he met a nurse who became his wife, Taimi, who died in 2014. They had three children, Karla, Tanya Evers and James Craig. His son died in 2009. He is survived by his daughters, three grandchildren and a great-grandchild.

Unable to return to the fighting, Leavelle became a civilian employee of the Army Air Force, running a military warehouse in Riverside, California. He then became an auditor for the federal government, investigating colleges receiving money under the GI Bill.

He spent his first six years on the Dallas force in patrol before making detective in 1956, and worked his way up from the burglary and theft squad to homicide, where he was working when Kennedy was assassinated on Nov. 22, 1963. Leavelle retired in 1976 and founded a polygraph business, which he turned over to his daughter Karla in 1980. He underwent triple-bypass heart surgery in 2004.

Leavelle, who remained active into his late 90s, traveled with the help of a Dallas police officer to the National Law Enforcement Museum in Washington in late 2018 to rerecord an oral history he had made several years earlier before the museum’s opening in October.

In his last years Leavelle served on the board of the Crime Stoppers organization in Garland, Texas, and continued giving talks to police and school groups, usually around the Nov. 22 commemorations. He was still making occasional appearances as late as 2018. He said that he received about 500 requests a year for his autograph — “more than I need” — and in a phone interview in December said he had “four or five on my desk” at the moment. He was particularly proud, he said, of an invitation to address the FBI’s graduation of its October 2006 agent training class at Quantico, Virginia.

As late as January he lived alone and unassisted. At 98 1/2, he fell while he was doing errands in Garland, but he got himself home and planned to see a doctor a few days later. “It’ll quit hurting in a little while,” he told a Times reporter by phone.

For years, the light-colored double-breasted suit that Leavelle wore in the famous photo gathered dust in his closet. He later lent it to the Sixth Floor Museum at Dealey Plaza, in the former Texas School Book Depository, from where Oswald is believed to have fired the fatal shots. It is displayed behind glass with his original hat, tie and handcuffs.

The boots on display are a later addition. He had thrown out the pair he was wearing on Nov. 22, 1963.

“I didn’t realize they were worth something,” he said.