Deep in north-eastern Poland, a neglected airstrip has been identified as a key staging post in the CIA’s clandestine torture programme.

For the first time, a detailed picture of how the CIA flew terror suspects into Szymany and on to a nearby ‘dark site’ for brutal interrogations has emerged.

In an exclusive interview with The Mail on Sunday, the airport’s former managing director Mariola Przewlocka reveals:

Scroll down for video

Ashamed: Mariola Przewlocka, who spoke exclusively to the Mail on Sunday, at the entrance to Szymany airport

Mysterious flights arrived with little notice – and up to six times the landing fees were paid by anonymous officials

Military cars with darkened windows took passengers from the plane in secret and off to a Polish military intelligence base

She believes she witnessed the arrival of September 11 ‘mastermind’ Khalid Sheikh Mohammed on a CIA Gulfstream jet – which later landed in Glasgow

Airport staff were forbidden to approach the aircraft

A quiet American woman, said to be ‘from the embassy’, once watched as a transfer took place

Basic safety rules were sometimes flouted.

Known by its codename ‘Detention Site Blue’, Szymany airport – which is 100 miles from Warsaw – was the destination for several terror suspects on unmarked civilian planes. They were hooded, handcuffed and shackled for ‘enhanced interrogation’ at nearby Stare Kiejkuty base.

The detainees would arrive, sometimes in the dead of night, in aircraft owned by CIA ‘shell companies’.

Some of the most brutal torture sessions in the CIA’s murky war against terror took place near this forbidding spot.

Mrs Przewlocka, who ran Szymany airport at the time, told of her shock at discovering she may have seen the arrival of the CIA’s most high-value prisoner, Khalid Sheikh Mohammed. Known as KSM to the CIA, he was waterboarded no fewer than 183 times by his captors, both during his six months in Poland and at other CIA facilities.

The Mail on Sunday has learned that the Gulfstream executive jet which ‘dropped off’ KSM in Poland then went on to stay overnight at Glasgow airport, where it stopped for 24 hours, presumably to allow the flight crew to rest.

The role of Szymany airport was highlighted in last week’s US Senate Intelligence Committee on CIA renditions. After being flown here, prisoners were transferred 13 miles on near-deserted roads to Stare Kiejkuty, where they were tortured.

Mrs Przewlocka realised the clandestine activity signified some kind of undercover operations being conducted but had no idea the facility was being used for ‘extraordinary rendition’.

The 57-year-old grandmother became suspicious after traffic to the airport suddenly picked up in late 2002. ‘The airport wasn’t doing well economically, operations were being run down,’ she recalled.

From December 2002, however, as President George W. Bush’s ‘war on terror’ escalated, the small planes she was used to seeing gave way to much bigger jets which thundered dangerously down the runway.

Szymany Airport, 100 miles from Warsaw, was known by codename Detention Site Blue

‘On one occasion the airport director told me a “special” flight was due to arrive the next day and it had to be given landing permission at any cost. I told him that wouldn’t be possible as there had been a lot of snow.

‘He said something like, “Don’t worry about that, bring in an outside contractor. However much it costs, we will pay”. When the plane touched down, it turned out to be an American-owned Gulfstream jet, which we’d never seen before at the airport. The customs staff were told to go home and a border police unit was brought in for the day, which was extremely unusual.

‘Two military cars from the intelligence base at Stare Kiejkuty drove up to the aircraft and after a few minutes returned to the airport building and then went out through the main gate. We couldn’t see what was going on because the cars had darkened windows. I assumed the flight was bringing in secret agents.’

The view of the tower on the airport in Szczytno

KSM told a US military tribunal he saw snow when the plane bringing him from Afghanistan stopped over in Europe. Mrs Przewlocka now believes she may have been at Szymany when the Al Qaeda terrorist’s plane touched down. She is unsure of the date, but independent records show his flight almost certainly arrived at Szymany on March 7, 2003. ‘I have my suspicions that this was the flight which we were under orders to accept at any cost,’ she said. ‘Everything was hidden from us.’

Mrs Przewlocka recalled a Polish civilian official who would always take care of landing fees in cash. ‘They would pay up to six times the normal charge for a civilian aircraft and we were instructed to keep away and ask no questions.’

The normal landing fee of around £380 could soar up to £2,300 for the flights, she said. On one occasion, Mrs Pzewlocka noticed a quiet American woman in the background when a flight came in. She recalled: ‘She was smartly dressed and didn’t speak to us but we were told she was from the American embassy. She waited near the office in the airport building and didn’t go near the planes. It was as if she didn’t want to know too much about what was going on.’

On September 22, 2003, a Boeing 737 was given permission to land at Szymany, although the runway was unsuitable for an aircraft of this size. Mrs Przewlocka said the flight plan indicated it had come from Kabul and was scheduled to refuel at Warsaw’s main civilian airport before going on to Guantanamo Bay. ‘This was inexplicable because if it could get to Szymany why couldn’t it fly directly to Warsaw which is only 100 miles or so away?

‘We should not have accepted the flight – there weren’t even any firefighters on duty, which is illegal – but we were given no choice. Once again, two military vehicles went out to meet it, waited for a few moments at the aircraft steps and then headed in the direction of Stare Kiejkuty. I saw several more 737s after that.’ The Senate report reveals the Polish authorities initially refused to allow KSM into the country, claiming they had accepted enough prisoners on behalf of the Americans already.

But their stance crumbled when the US ambassador personally intervened with the government in Warsaw, followed by a CIA delivery of $15 million in cash, after which Polish officials assured the Americans they would be more flexible.

Research by the Rendition Project, a collaboration between academics at Kent and Kingston universities, has pieced together the journey followed by the plane almost certainly carrying KSM – a Gulfstream V jet, code-number N379P and owned by a CIA company.