WARSAW — For years, the idea seemed unthinkable, absurd. A secret U.S. detention center in a remote corner of Poland, where al-Qaida suspects were brutally interrogated by the CIA? About as likely as “the Loch Ness monster,” is how one Pole described it recently.

That monster is now rearing its head.

Cloistered inside government offices, surrounded by classified documents, Polish prosecutors are building a case that could result in criminal charges against the nation’s former spy chief and even, some say, against former senior political leaders. Evidence that a foreign power was allowed to conduct illicit activities on Polish soil has deeply shaken many Poles’ faith in the United States and in Poland’s sense of itself as a successful democracy born from the ashes of the Cold War.

PROBE FOCUSES ON GARRISON

The prosecutors’ investigation centers on a Polish military garrison that allegedly hosted a CIA “black site” where foreign detainees were subjected to internationally condemned interrogation techniques, such as waterboarding, during 2002 and 2003.

The suspects — including Khalid Sheikh Mohammed, the self-professed mastermind of the Sept. 11 attacks — had either been arrested or snatched under the United States’ “extraordinary rendition” program and questioned abroad to avoid American legal standards for interrogations, prosecutors say.

The allegations have already damaged the reputation of the country that Poles thank for helping them to cast off communist oppression. Many now angrily believe the U.S. took advantage of their gratitude, loyalty and eagerness to please by setting up a torture site that it would never have allowed within its own borders.

“It’s the kind of thing we expect from Soviet Russia. We remember the Soviet occupation; we remember the German occupation,” said attorney Mikolaj Pietrzak, who represents one of the Islamist men allegedly held and questioned in Poland. “The fact that this beacon of liberty which is America would allow this – it’s a great disappointment in the United States as the land of the free.”

OTHER NATIONS MAY HAVE HOSTED SITES

Poland is not the only country in Europe where the U.S. allegedly operated a secret detention facility with at least tacit permission from somewhere within the host government. Black sites are also thought to have existed in Romania and Lithuania, two other developing democracies, as well as in countries in North Africa and Asia.

But Poland is alone among the European nations in having launched an official investigation of the matter.

“The reputation of Poland is at stake,” President Bronislaw Komorowski declared in March. “Certainly this is a sensitive and touchy issue, and possibly painful for the Polish state, but it is the task of the legal apparatus to clarify this.”

At the same time, a number of lawyers, journalists and rights activists complain that the investigation has been halting, opaque and prone to political meddling because of its potential repercussions for U.S.-Polish relations and for prominent public figures who may have known about the suspected CIA site.

INVESTIGATION TRADES HANDS

The case has traded hands in the national prosecutor’s office at least twice since the investigation began in 2008. Recently, for reasons that are unclear, it was transferred from the office in Warsaw to the southern city of Krakow.

Pietrzak is frustrated by prosecutors’ refusal to give him access to classified files beyond the initial perusal he was granted.

His client, Abd al-Rahim al-Nashiri, is accused of plotting the 2000 attack by al-Qaida on the U.S. destroyer Cole in Yemen, which killed 17 American sailors. Captured in late 2002 in the United Arab Emirates, al-Nashiri, a Saudi national, is now an inmate at the U.S. military detention center in Guantanamo Bay, Cuba.

During his interrogation at the suspected CIA black site in northern Poland in 2002 and 2003, at a military base in the northeastern town of Stare Kiejkuty, a gun and a power drill were allegedly pointed at al-Nashiri’s head to make him talk.

His lawyer says he was also subjected to waterboarding, a simulated drowning technique that the U.S. has since banned.

Al-Nashiri now awaits trial before a U.S. military tribunal and could face the death penalty, which makes Pietrzak chafe all the more at the pace of the prosecutors’ effort here in Poland.

“It’s not a robust investigation if it takes you four years,” Pietrzak said. “This is the single worst case of human rights violations known in Eastern Europe in the last 20 years. The public has a right to know.”

What the public knows so far has been due in large measure to the dogged work of journalists such as Adam Krzykowski, a reporter for Polish television.

While allegations here of a secret U.S. interrogation site in Stare Kiejkuty were still being dismissed in Poland as fantasy four or five years ago, Krzykowski obtained the flight logs of several jets that landed at nearby Szymany airport in 2003, a facility usually frequented by small private planes carrying visitors to the scenic region.

Mohammed was captured in Pakistan in 2003. He has said he believes he was interrogated in Poland because, during questioning, he was given a bottle of water whose label bore an email address ending in “.pl,” the Internet country code for Poland.

By 2008, the weight of evidence and public allegations was such that Polish prosecutors felt compelled to launch their own investigation. The country’s president, prime minister and other senior officials at the time the secret prison is alleged to have been in operation have all denied knowledge of any black site on Polish soil.