"The demeanour of people in the one line seemed relaxed. The people in the other line were heavily guarded. They seemed stunned, stolid-faced. We never saw those people again."
In September 1973, Mr Schesch was held prisoner for 10 days in the bowels of Santiago's national stadium as the madness and evil of a military coup played out in front of him.
No one knows how many ordinary citizens lost their lives after General Augusto Pinochet turned on his friend, President Salvador Allende, and ruthlessly seized power. But, in the 10 days he was held, Mr Schesch believes that between 400 and 600 people were executed by firing squads just yards from where he sat, hunched and terrified with his wife. Among the victims were fellow US citizens Charles Horman and Frank Teruggi.
"Just before they led the one line out into the stadium, they would start the extractor fans in the changing rooms just to make some noise," he said. "They did not want people to know what was happening. Then they would lead the line of people out of the changing rooms into the stadium."
Moments later, Mr Schesch would hear the unmistakable sound of machine- gun fire.
"There was a concrete wall between where I was being held and the stadium but I could hear everything, said Mr Schesch. "On one occasion my wife heard the people in the stadium singing `the Internationale'. Then the machine-guns started.
"The gunfire would last for 45 seconds, maybe a minute, and then there was no sound. Then someone would come back in and turn off the fans."
Twenty-five years later, Mr Schesch, 55, is remarried with a grown-up son and working as researcher for the State of Wisconsin. But the arrest in London last week of General Pinochet - the alleged architect of the massacre in the stadium - has stirred memories he will never forget.
He and his former wife, Patricia, had travelled to Chile in December 1970, arriving the day after President Allende was sworn in. They remember it as an exciting time.
The two graduates were rapidly swept up in the euphoric lifestyle of working for an avowedly left-wing democracy in a continent teeming with military dictators. They worked with a Catholic University in Santiago, mixed with the local activists and became heavily involved in Allende's government of Popular Unity.
"Once we got there we realised it was a historical moment. It was incredibly exciting. That first year was incredibly upbeat and promising," he said. "Then after 18 months things became incredibly grim."
He remembers the strikes, the boycotts, the struggle for control in the elections of March 1973 and the attempted coups. Then on the morning of 11 September, they received a phone-call. "It was a friend. He said there had been a coup and we should put on the radio immediately."
The news was stark and to the point. Allende had committed suicide as British-built Hawker jets bombed his palace, the military had seized power and foreigners were being denounced as killers. "There was nothing we could do but stay in our house and try and burn the pamphlets and papers we had. We only had a small kerosene fire and we had to burn everything on that in small bundles at a time. The fire burnt for three days."
Shortly afterwards, Mr Schesch and his wife were denounced by a neighbour and taken by police to the national stadium where the authorities were rounding people up and holding them. In the changing rooms underneath the main building they were splitting people into different categories. Some rooms contained intellectualsy, others members of the working class and native Indians.
Mr Schesch had no idea what would happen to him and his wife. They had tried to get rid of anything that could link them to the Allende government and stressed their status as American citizens. "We were totally unprepared. We did not know what to do. They basically just threw us down against a concrete wall where we were held for 10 days while they questioned us. If we wanted to go to the toilet they took us to a room containing about 75 people," said Mr Schesch.
"We saw people being beaten up and taken off for torture. At one point they brought in two teenagers who had been fighting against the anti-government forces and they beat them up in front of us."
It was during the first three days they were held captive that Mr Schesch and his wife became aware of the lines of people being formed up and then led out of the stadium - almost certainly to concentration camps - or else inside, to face the firing squad.
"They were taking people into the stadium about 20 at a time, several times a day. It was men and women. Mainly adults. Mainly working-class people. There were a lot of native Indians."
Mr Schesch was eventually released after friends put pressure on the American State Department to intervene. There had been reluctance, possibly because - as recently released documents show - the CIA acting under President Richard Nixon was deeply involved in promoting the coup.
After 10 days, he and his wife were handed over to someone from US Embassy. The following morning they were flown back to the States. They have never been back to Chile.
"The memories of that time are very vivid. Those things stay in your memory. I had dreams for several years afterwards and I suffer - even now - from a feeling of vulnerability," said Mr Schesch. "Chile was meant to be the Switzerland of South America. What we saw was the brutal destruction of democracy."
In 1991, when a number of graves of coup victims were dug up, it was revealed that many of the bodies had broken arms and legs, indicating torture.
It was also revealed that many victims were buried two-to-a coffin. "What a great saving," General Pinochet is reported to have said. "I congratulate the diggers."
Such comments add to Mr Schesch's conviction that Britain has a duty not to give in now to Chile's demands for the release of General Pinochet. He believes even at the age of 82 he should be handed over to the Spanish judges requesting his extradition and tried for war crimes.
"It was not genocide but `politicide' what he was doing. He was trying to wipe out the leadership of a whole generation of the working class," he said.
"I urge the people of Britain to stand firm and not be overwhelmed. It is for a generation of people and to the children of that generation."