ISTANBUL (NYTIMES) - When Turkish police officers stopped him as he set out for the airport to return to the United States after a family vacation in Turkey, the country of his birth, Serkan Golge, a Nasa scientist and American citizen, was in disbelief.

It was July 2016, eight anxious days after a failed coup tried to unseat President Recep Tayyip Erdogan, and police told Mr Golge that they had received an anonymous tip that he worked for the CIA and was part of a terrorist group accused of masterminding the plot.

The idea was so far-fetched that Mr Golge expected to sort it out quickly and changed his flight to the next day.

"I was quite shocked, but I was like, 'This will go away,' " he said.

"This is probably a mistake, and the police and prosecutors would figure this out."

It would take four years.

Mr Golge and his family returned to Houston just last week, ending a nightmare in which he was held for three years in solitary confinement as he became a bargaining chip in a series of high-level disputes between the Turkish and US governments.

In his first interview since arriving home, Mr Golge described with exasperation but little rancour the ordeal of being charged and found guilty of terrorist activities on evidence so flimsy he called it "garbage."

His account provides a rare insight into the Turkish judicial machine from the side of a defendant.

Some 70,000 people have been accused in the Turkish courts in connection with the failed coup.

Many, still fearful of the whims of Turkish justice, prefer to keep silent even once they are freed.

Mr Erdogan's government has blamed the coup attempt on Fethullah Gulen, a Muslim cleric who lives in Pennsylvania.

Soon after the coup, pro-government Turkish media outlets began accusing the US government of being behind the plot, suggesting that it was in league with Mr Gulen.

For Mr Golge, who has a doctorate in physics and worked as a senior research scientist at Nasa's Johnson Space Centre, being an American citizen was enough to be presumed guilty.

"You fit the profile," he recalled his lawyer telling him at one point.

"It does not really matter if you are innocent or not. They won't release you."

After 14 days, Mr Golge appeared before a judge who told him police had found an American dollar bill in his parent's house, which Turkish authorities alleged was a badge of membership to the Gulen movement, by then designated a terrorist group.

He was held in a general prison in southern Turkey, alongside high-ranking military officers, judges and prosecutors, some of whom told him that they were being held without any evidence at all.

Senior military officers and civilian supporters of Mr Gulen have been charged over their part in leading the coup and ordering the bombing of the Parliament and clashes that killed 250 people.

But thousands of others who were accused had only tenuous links to Mr Gulen's movement, or, like the military cadets who were ordered out on the night of the coup, had little idea what was going on.

Journalists and political opponents of Mr Erdogan with no connection to the events were prosecuted as well.

Mr Golge was sent to a prison in the town of Iskenderun, where in the August heat 32 men were crammed in a cell made for eight.

He slept on a blanket on the floor and soon fell ill with bronchitis.

Within a month, he was moved to solitary confinement and faced charges of overthrowing the government and the constitution, which carried a life sentence, and a charge of belonging to a terrorist organisation, which carried a 15-year sentence.

"That's it; I'm never getting out of here," he recalled thinking. "That was a collapse psychologically, and I cried a lot."

"It is a very small room - it barely sees the sunlight, and the guards took me out only one hour a day," he said.

"And I stayed in that room, in that small single cell, for three years."

For a long time, Mr Golge clung to the fact that the evidence Turkish prosecutors presented was hardly incriminating.

The anonymous tip turned out to be from a relative who bore a grudge against Mr Golge's sister and later admitted he did not know if his allegations were true.

The prosecutors drew on other evidence, and even Mr Golge acknowledges that he fits the profile of a possible member of the Gulen movement.

He went to Fatih University, which was one of the most prominent Gulen schools, on a scholarship to study physics; he banked with Bank Asya, which was part of the Gulen network of companies.

But none of that, he points out, amounts to a crime.

"A one dollar bill, an anonymous tip, a bank account? How is this terrorism?" Mr Golge asked.

"Nobody could explain, but I think this is how laws and courts still work in Turkey."

Mr Golge has condemned the coup attempt and says he had nothing to do with the Gulen movement.

"I am not part of this organisation," he said.

"I am very sorry for the people who lost their lives. This is something unacceptable. Violence is never a solution. I have always believed in democracy, and I think currently it is the best solution we have."

But he says Turkey missed an opportunity by not dealing justly with the coup attempt.

Instead, zealous prosecutors have pursued people far beyond the actual perpetrators, sweeping up many who have been judged guilty by association.

Mr Golge recalls a fellow inmate, a former judge, telling him the government had no evidence against him.

"At least there was some bogus evidence about you," he told Mr Golge, "but I don't know why I was arrested."

He shared time in the exercise yard with a one-star general who told him he had opposed the coup but had been convicted and sentenced to life imprisonment anyway because his name appeared on a list of appointments made by the coup plotters.

"If Turkey only prosecuted the responsible people, instead of prosecuting hundreds of thousands of innocent people, I think that Turkish democracy would come out of this horrible act much stronger," Mr Golge said.

Gradually, with American officials including President Donald Trump pressing Turkey for his release over the lack of evidence, the charges against Mr Golge were reduced.

He was eventually convicted of aiding a terrorist organisation, and the sentence was reduced on appeal.

He said he sensed the Turkish judges knew the case against him was "garbage" but were compelled to drag out the process.

"I felt they were scared of something," he added.

He was released from prison in May 2019 and in April this year was cleared to leave the country.

But then he was hospitalised with stomach ulcers, and the coronavirus pandemic grounded flights.

The strain of the past four years on his wife, Kubra, and two boys, ages 4 and 10, erupted at the airport, when Mr Golge was pulled aside at passport control and held for 40 minutes.

"My wife started crying; the kids started crying," he said.

"I tried to stay calm because I knew they had no basis to hold me, but they were shaking so hard. My son was crying a lot, grabbing me, holding on to me, saying 'No Dad, not again, not again.'"

Officials at the US Embassy, who were tracking his progress, ensured he made the flight.

Back in Houston, he is rebuilding his life, applying for his old job and looking for a house.

"Your life - four years, three years in prison - will not come back," he said.

"But that's life. Sometimes you lose; sometimes you win."