The austere building stands in the middle of a remote, thick forest surrounded by a high fence and numerous surveillance cameras.
The two-story structure _ a former riding school for the rich _ is allegedly where the CIA held suspected Al-Qaida prisoners five years ago.
Lithuanian officials have denied that the former Soviet republic _ a close U.S. ally in the war on terror _ hosted clandestine detention centers. But after persistent reports in local and international media, a parliamentary committee launched an investigation last month.
Lawmakers investigating the claims said they visited the site last week, but declined to give details pending the outcome of the probe.
"We do not publicly disclose or comment on data accumulated during the committee's investigation until it is closed and conclusions are made," committee chairman Arvydas Anusauskas said Thursday.
The former horseback riding school, built in 1992, was privately owned until 2004 when it was sold to Elite L.L.C., a company that no longer exists. The State Security Department bought it in 2007.
Situated on the outskirts of the small village of Antaviliai, 12 miles (20 kilometers) northeast of the capital, Vilnius, it would be a perfect spot for a secret jail. It is remote but less than an hour's ride from the international airport.
Lithuania's security police use it as a training center. It has no street address and uses a post office box number in the village.
On Thursday evening, several cars were parked outside the dark building. Guards patrolled the fence. The paved driveway, lined by neatly cut grass and basketball courts, stood in sharp contrast to neglected buildings nearby.
Villagers said they remembered foreign workers at the site years ago.
"We are used to hearing that Lithuanians work abroad, not the opposite," said a 70-year-old woman who lives in the village. She declined to give her name. "Everything was quiet, well-organized but the only strange thing was that foreigners worked there," she said.
A neighbor who lives in an apartment a few hundred yards (meters) away, said he remembers hearing English-speaking workers "digging and removing large amounts" of earth.
"I thought to myself; that must be a luxurious sauna with a swimming pool underground they are building," the man said. He too refused to give his name. Lithuanians, especially those living in the countryside, tend to be shy and wary of having their names appear in print.
In Washington, the CIA declined comment on the alleged Lithuanian agency prison, also known as a "black site."
As one of his first acts in office, U.S. President Barack Obama ordered the closure of all Bush-era CIA secret prisons and ended the use of harsh interrogation tactics.
An ABC News report in August said the CIA had a secret prison in Vilnius from September 2004 through November 2005, and used it to detain and interrogate al-Qaida prisoners captured around the world after the Sept. 11, 2001 terror attacks. On Wednesday, ABC said the "black site" had been used as a prison by the agency, citing an unnamed Lithuanian official.
Lithuanian Prime Minister Andrius Kubilius said he was surprised by the reports.
"It is a mystery to me, but I believe our parliamentary probe will answer all questions," he said.
Foreign Minister Vigaudas Usackas said the government would "follow hard facts rather than rumors and wild tales."
Lawmakers called for the probe after President Dalia Grybauskaite met top European human rights officials Oct. 21.
"I have indirect suspicions _ and not only me, but the entire international community," Grybauskaite said after the meeting. "If there was such a thing, Lithuania should come clean, take responsibility, and promise that it will never happen again."
In a 2007 probe conducted on behalf of the Council of Europe, Swiss senator Dick Marty accused 14 European governments of permitting the CIA to run detention centers or conduct rendition flights through their countries between 2002 and 2005.