The alarm sounded during one of the tensest periods in the Cold War. Three weeks earlier, the Soviets had shot down a Korean Air Lines 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 V. Andropov, was obsessed by fears of an American attack.

Colonel 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 Mr. 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 — Colonel 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.

Colonel 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. It was confirmed by his son, Dmitri, according to Karl Schumacher, a political activist who, after learning in 1998 of Colonel Petrov’s Cold War role, traveled to Russia to meet him and remained a friend. The cause was hypostatic pneumonia.