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It was a fraction of a second that jolted Americans' view of the Vietnam War.

In a Saigon street, South Vietnam's police chief raised a gun to the head of a handcuffed Viet Cong prisoner and abruptly pulled the trigger. A few feet away, Associated Press photographer Eddie Adams pressed his shutter.

Taken during the North's surprise Tet Offensive, Adams' Feb. 1, 1968, photo showed the war's brutality in a way Americans hadn't seen before. Protesters saw the image as graphic evidence that the U.S. was fighting on the side of an unjust South Vietnamese government. It won Adams the Pulitzer Prize. And it haunted him.

"Pictures don't tell the whole story," he said later. "It doesn't tell you why."

After 50 years, the Saigon execution remains one of the defining images of the war. Time magazine has declared it one of history's 100 most influential photos.

"It still represents a lot of what photojournalists do, that idea of bearing witness to an important event," says Keith Greenwood, a University of Missouri photojournalism-history professor. "There are ugly things that happen that need to be recorded and shared."

It was the second day of the Tet Offensive. North Vietnamese forces and Viet Cong guerrillas had attacked South Vietnamese towns and cities, including the capital, Saigon, during a holiday cease-fire.

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Adams, a former Marine Corps Korean War photographer who joined the AP in 1962, and NBC cameraman Vo Suu had been checking out fighting in a Saigon neighborhood when they saw South Vietnamese soldiers pulling a prisoner out of a building, toward the newsmen.

The soldiers stopped. The police chief, Lt. Col. Nguyen Ngoc Loan, walked up and lifted his pistol. Adams figured the chief planned a gunpoint interrogation.

South Vietnamese forces escort Bay Lop moments before his execution. Eddie Adams / AP

Instead, Loan fired, and Adams' photo froze prisoner Bay Lop's grimace as he was shot. Suu's footage also captured the moment, in motion.

Loan told the two: "They killed many of my men and many of your people" and walked away, Adams recalled in a 1998 interview for an AP oral history project.

At the AP's New York headquarters, photography director Hal Buell saw the image emerging from the radio-based system used to transmit photos at the time. After some deliberation, he and other editors decided to distribute it worldwide.

"I knew when it went out that you were going to get two reactions. The doves were going to say, 'See the kind of people we're dealing with here (in South Vietnam)?' And the hawks said, 'It shouldn't have been used — you guys gotta get on the team,'" says Buell, now retired.

But "the image had an impact, and its impact was felt by those people who were on the fences."

The photo appeared on front pages, TV screens and protest placards. The Tet Offensive proved a military failure for the Communists, but it fueled the American public's pessimism and weariness about the war. It ended when the North prevailed in 1975.

Adams, meanwhile, felt Loan was unfairly vilified by a public that didn't see something outside the frame: the killings of Loan's aide and the aide's family hours earlier by the Viet Cong.

"I don't say what he did was right, but he was fighting a war, and he was up against some pretty bad people," Adams said. He rued that "two people's lives were destroyed that day" — Lop's and Loan's — "and I don't want to destroy anybody's life. That's not my job."

Loan died in 1998 in Virginia, where he ran a restaurant. Lop's widow told the AP in 2000 that she felt the picture helped turn Americans against the war.

Adams, who died in 2004, was more proud of his 1977 photos of people fleeing postwar Vietnam. Those images helped persuade the U.S. government to admit over 200,000 of the refugees (one of the pictures also is on Time's 100-most-influential list). His legacy includes the annual Eddie Adams Workshop for emerging photojournalists, which marked its 30th year this fall.

Work and fundraising are underway to expand a 2012 short documentary about the famous photograph, "Saigon '68," into a full-length film.

Director Douglas Sloan says it will encourage people to understand the context of what they see in powerful images.