NEW YORK (NYTIMES) - Early on the morning of Sept 26, 1983, Stanislav Petrov helped prevent the outbreak of nuclear war.

A 44-year-old lieutenant colonel in the Soviet Air Defence Forces, he was a few hours into his shift as the duty officer at Serpukhov-15, the secret command centre outside Moscow where the Soviet military monitored its early-warning satellites over the United States, when alarms went off.

Computers warned that five Minuteman intercontinental ballistic missiles had been launched from a US base.

"For 15 seconds, we were in a state of shock," he later recalled. "We needed to understand, 'What's next?'" The alarm sounded during one of the tensest periods in the Cold War.

Three weeks earlier, the Soviets had shot down a Korean Airlines commercial flight after it crossed into Soviet airspace, killing all 269 people on board, including a congressman from Georgia.

President Ronald Reagan had rejected calls for freezing the arms race, declaring the Soviet Union an "evil empire." The Soviet leader, Yuri Andropov, was obsessed by fears of a US attack.

Petrov was at a pivotal point in the decision-making chain. His superiors at the warning-system headquarters reported to the general staff of the Soviet military, which would consult with Andropov on launching a retaliatory attack.

After five nerve-racking minutes - electronic maps and screens were flashing as he held a phone in one hand and an intercom in the other, trying to absorb streams of incoming information - Petrov decided that the launch reports were probably a false alarm.

As he later explained, it was a gut decision, at best a "50-50" guess, based on his distrust of the early-warning system and the relative paucity of missiles that were launched.

Petrov died at 77 on May 19 in Fryazino, a Moscow suburb, where he lived alone on a pension. The death was not widely reported at the time and has only now been made public.

It was confirmed by his son, Dmitry, according to political activist Karl Schumacher.

Schumacher learned in 1998 of Petrov's Cold War role and travelled to Russia to meet him and remained a friend. The cause was hypostatic pneumonia.

Stanislav Yevgrafovich Petrov was born on Sept 7, 1939, near Vladivostok, Russia. His father had been a fighter pilot during World War II; his mother was a nurse. He studied at the Kiev Higher Engineering Radio-Technical College of the Soviet Air Force.

After joining the Air Defence Forces, he rose quickly through the ranks; he was assigned to the early-warning system at its inception in the early 1970s.

Historians who have analysed the episode say that Petrov's calm analysis helped avert catastrophe.

As the computer systems in front of him changed their alert from "launch" to "missile strike," and insisted that the reliability of the information was at the "highest" level, Petrov had to figure out what to do.

The estimate was that only 25 minutes would elapse between launch and detonation.

"There was no rule about how long we were allowed to think before we reported a strike," he told the BBC. "But we knew that every second of procrastination took away valuable time, that the Soviet Union's military and political leadership needed to be informed without delay. All I had to do was to reach for the phone; to raise the direct line to our top commanders - but I couldn't move. I felt like I was sitting on a hot frying pan."

As the tension in the command centre rose - as many as 200 pairs of eyes were trained on Petrov - he made the decision to report the alert as a system malfunction.

"I had a funny feeling in my gut," he told The Washington Post. "I didn't want to make a mistake. I made a decision, and that was it." Petrov attributed his judgment to both his training and his intuition.

He had been told that a nuclear first strike by the Americans would come in the form of an overwhelming onslaught. "When people start a war, they don't start it with only five missiles," he told The Post.

Moreover, Soviet ground-based radar installations - which search for missiles rising above the horizon - did not detect an attack, although they would not have done so for several minutes after launch.

Petrov was at first praised for his calm, but in an investigation that followed, he was asked why he had failed to record everything in his logbook. "Because I had a phone in one hand and the intercom in the other, and I don't have a third hand," he replied.

He received a reprimand.

The false alarm was apparently triggered when the satellite mistook the sun's reflection off the tops of clouds for a missile launch. The computer program that was supposed to filter out such information had to be rewritten.

Petrov said the system had been rushed into service in response to the United States' introduction of a similar system. He said he knew it was not 100 per cent reliable.

"We are wiser than the computers," he said in a 2010 interview with the German magazine Der Spiegel. "We created them."

Cold War tensions persisted. In November 1983, Nato carried out Able Archer 83, a big military exercise simulating a coordinated nuclear attack. The exercise, alongside the arrival in Europe of Pershing II nuclear missiles, led some in the Soviet leadership to believe that the United States was using it as a cover for war; the Soviets placed air units in East Germany and Poland on alert. (Able Archer is the backdrop for the recent television series "Deutschland '83.")

Petrov retired from the military in 1984. He got a job as a senior engineer at the research institute that had created the early-warning system, but retired to care for his wife, Raisa, who had cancer. She died in 1997.

In addition to his son, Dmitry, Petrov is survived by a daughter, Yelena, and two grandchildren.

Petrov had largely faded into obscurity - at one point he had been reduced to growing potatoes to feed himself - when his role in averting nuclear Armageddon came to light in 1998 with the publication of the memoir of Gen Yuriy Votintsev, the retired commander of Soviet missile defense.

The book brought Petrov a measure of prominence. In 2006, he travelled to the United States to receive an award from the Association of World Citizens, and in 2013 he was awarded the Dresden Peace Prize. He was the subject of a 2014 hybrid documentary-drama, "The Man Who Saved the World."

Jakob Staberg, the producer of the film, said in a phone interview Monday that he had tried to contact Petrov by phone and email for the last several weeks, hoping to discuss the film's Russia release, scheduled for February. He said he had not thought much of the delay because Petrov often traveled.

Petrov's role in the film brought him into contact with American celebrities like actors Kevin Costner and Robert De Niro, but he did not embrace the spotlight. "I was just at the right place at the right time," he says in the film.