Warning - graphic content and disturbing image below
Before he pulls the trigger, the South Vietnamese general waves his soldiers out of the way so they don't get hurt.
He shoos them with the pistol he holds in his right hand, and flicks out his left hand as he approaches the prisoner standing before him.
His men guess what's coming, and scatter. The general raises the shiny snub-nose .38, points it at the prisoner's right temple and pulls the trigger.
At that instant on the sunny Thursday of February 1, 1968, in what was then called Saigon, Associated Press photographer Eddie Adams' camera shutter clicked once and one of the most powerful pictures of the Vietnam War, or any war, was taken.
In 1/500th of a second, Adams caught the moment the bullet crashed through the Viet Cong prisoner's skull at about 965kmh, distorting his face, tousling his hair and shoving his head off centre.
Some people say you can see the bullet coming out the other side. Adams thought it was still inside. It's hard to tell.
In the picture, General Nguyen Ngoc Loan, the head of South Vietnam's National Police, stares coldly at his victim. Loan, who had a reputation as a government enforcer, is a thin man with a receding hairline who seems to be wearing a bulky flak vest.
His shirt sleeves are rolled up, showing the sinews in his forearm. The gun looks like it has recoiled slightly upward. Off to the left in the frame, another soldier winces.
The prisoner, whose hands are bound behind him, already has a fat lip, likely acquired when he was captured after leading a brutal commando raid. He wears dark shorts, a plaid shirt and no shoes.
On film footage of the incident, the shooting scene unfolds in about 10 seconds. (Adams can be seen for an instant in an earlier part of the footage.)
The prisoner, Viet Cong officer Nguyen Van Lem, tumbles to the pavement, blood spouting from his head. Loan calmly holsters his pistol, saying to Adams, "they killed many of my men and many of your people," and walks away.
Adams waits until the blood stops gushing. "It was gross," he would say later. He takes a few more pictures of the body then returns to the Associated Press office to drop off his film.
No big deal.
"I thought absolutely nothing of it," he said in an interview that later became part of a Newseum podcast.
"I said 'I think I got some guy shooting somebody.' And, uh, I went to lunch."
Just another day on the bloody streets of Saigon in the midst of the enemy's famous Tet offensive - sweeping guerrilla attacks across South Vietnam - during the endless Vietnam War.
"So what?" Adams, a former US Marine from a small town north of Pittsburgh, said later.
"It was a war. I'm serious. That's how I felt. I had seen so many people die at that point in my life."
Adams did not realize he had taken one of history's great pictures. He did not know it was a shot that would summarise in a millisecond the savage, seemingly mindless, violence of the war.
He had no idea that his photograph, snapped 50 years ago Thursday, would help change history, and echo throughout his life and that of his surviving subject.
"He wasn't allowed to forget that photograph," Loan's son, August, said in a 2008 documentary. "It stuck with him everywhere he went."
The picture ran on the front pages of many US newspapers and the footage ran on TV. But it was the photograph, and its frozen portrait of agony, that fueled the antiwar movement and helped end US involvement.
"It just kind of summed up the whole war," former CBS News anchor Bob Schieffer said in the short film "Eddie Adams: Saigon '68" by Douglas Sloan.
"The horrificness of it stops you," the late Life Magazine photographer Bill Eppridge said in the 20128 documentary.
"I think his picture was the moment that changed the war."
David Hume Kennerly, a fellow Vietnam War photographer, who was the former chief White House photographer and a friend of Adams's, compared the picture with that of Joe Rosenthal's famous shot of US Marines raising the flag on Iwo Jima in World War II.
"Where Joe Rosenthal's picture represented the heroism . . . and courage in war and patriotism . . . Eddie's picture was exactly the opposite," Kennerly said in a telephone interview.
"Eddie's picture was the real underbelly of violence and summary execution . . . It's what war is really like."
"I don't know that it ended the Vietnam War, but it sure as hell didn't help the cause for the government," he said.
"One thing I know for sure, anybody who's ever seen that photo has never forgotten it."
In 1969, Adams was awarded a Pulitzer Prize for the shot.
But he felt terrible about the photo, which he didn't think was that good, and bitter about the prize, according to interviews he gave over the years.
He believed he had taken far more worthy pictures, and that the execution photo was viewed out of context by most people: The slain Viet Cong prisoner was suspected in the murder of a South Vietnamese officer, his wife and children.
Adams believed he had destroyed General Loan's life.
"Two people died in that photograph," Adams wrote in Time magazine years later.
"The recipient of the bullet and General Nguyen Ngoc Loan. The general killed the Viet Cong; I killed the general with my camera."
"General Loan was . . . a real warrior," Adams wrote in a Time eulogy for Loan.
"I'm not saying what he did was right, but you have to put yourself in his position . . . He never blamed me. He told me if I hadn't taken the picture, someone else would have, but I've felt bad for him and his family for a long time."
Loan, who later lost a leg in combat, was treated for the injury at Washington's old Walter Reed Army Hospital in 1969, which outraged some people. The late Senator Stephen Young, D-Ohio, called Loan a "brutal murderer," and his treatment in the U.S. "a disgraceful end to a . . . disgraceful episode."
Loan was university-educated and had become a jet pilot before he was named national police chief. He was married and had five children. After the war, he made his way with his family to the United States and ran a restaurant in northern Virginia. But the photograph stalked him. In the restaurant men's room, someone scrawled on the wall: "We know who you are, you f----"
In 1976, he told a Washington Post reporter he was trying "to think about the present and the future of my children. I have no time to think back or regrets."
In 1978, the government moved to deport him.
"Gen Loan cold-bloodedly shot and killed another human being," former Rep. Elizabeth Holtzman, D-N.Y., wrote at the time. "By any standard what he did was immoral."
But Loan had local support and was never deported. Twenty years later, on July 14, 1998, he died at home in Burke, Virginia, at the age of 67.
Adams, a rough-edged, cantankerous figure who had great fame after the war, also was haunted by the photo, among other things.
In the documentary, he said that when he died he wanted to be buried in his Marine Corps "dress blues" uniform. He wanted a 35mm camera with a wide-angle lens, and he wanted some slow speed colour film.
"Because where I go there's going to be a lot of light," he said.
"And it (won't be) from up above either. Fire, you can photograph really well with a slow speed film."
Adams died of Lou Gehrig's disease on September 18, 2004, in New York City. He was 71. He was laid to rest beside his father just outside his home town of New Kensington, Pa. He didn't fit into his old uniform, his widow, Alyssa, said. But she did bury him with a camera.