But what made it suspicious was how loud and clear the alert was.

"There were 28 or 29 levels of security. After the target was identified, he had to go through all these 'checkpoints'. I wasn't sure if that was possible under the circumstances," says the retired officer.

Petrov called the duty officer at Soviet Army headquarters and reported a system failure.

If he was wrong, the first nuclear explosions would have occurred minutes later.

"Twenty-three minutes later I realized that nothing had happened. If there had been a real attack, then I would have known about it. It was a huge relief," he says with a smile.

"Lucky it was me"

Soviet protocol stated that the military should respond to one nuclear attack with another.

Now, 30 years later, Petrov believes the chances were 50-50. He admits he was never entirely sure the alert was false.

He says he was the only officer on his team who had received a civilian education. "My companions were professional soldiers, they were taught to give and obey orders," he said.