A U.S.-based human rights group said Wednesday it suspects the CIA had a secret prison in a third European country and it plans to investigate.

Human Rights Watch did not identify the third country but said it was one of the 25 members of the European Union. Previously, the group identified Poland and Romania as sites of possible secret detention centers run by the United States.

"We do have one other individual allegation of a country, a country that is a member of the European Union," Lotte Leicht, director of the group's Brussels-based office, told the European Parliament's Subcommittee on Human Rights on Wednesday. "We are trying to conduct a surprise visit there."

Leicht refused to give more details, fearing it would compromise the investigation. But she told The Associated Press the New York-based human rights watchdog considered its source "very credible."

"We have lots of information from intelligence sources that are so far not willing to go public. We know that if they do, they will ... lose their jobs," Leicht told the committee.

Allegations that the CIA hid and interrogated key Al Qaeda suspects at Soviet-era compounds in Eastern Europe were first reported Nov. 2 in The Washington Post. After the report appeared, Human Rights Watch said it had circumstantial evidence indicating the CIA transported suspected terrorists captured in Afghanistan to Poland and Romania.

Both countries have repeatedly denied any involvement and EU officials say nothing has been proven. Poland said last week it had closed its investigation into allegations that the country may have hosted secret CIA prisons, but did not release any findings.

Secretary of State Condoleezza Rice has refused to address questions about clandestine CIA detention centers, but says the United States acts within the law.

Leicht urged the European Parliament to pressure member governments to help obtain details from secret services on the alleged detention centers.

"Getting information from those sources is almost impossible," she said.

The EU plans to launch its own probe into the issue later this month.