One of two prosecutors investigating whether Poland allowed a secret CIA prison to operate on its territory was replaced in what officials insisted Tuesday was an administrative shuffle.

The criminal investigation, launched in 2008, is trying to determine whether Polish leaders abused their powers by allowing Americans to operate the secret CIA site.

But some human rights activists expressed concern that the decision to replace prosecutor Jerzy Mierzewski could slow down or even completely derail the investigation.

The case is being closely watched by human rights groups because Poland is the only country known to be investigating the secret CIA operation that spirited terrorism suspects across the globe and subjected them to harsh interrogation techniques after Sept. 11, 2001. A similar investigation in Lithuania was closed early this year before all evidence was considered.

A leading human rights lawyer criticized the decision as "irrational."

"There was no objective reason for removing Mierzewski," said Adam Bodnar, a lawyer with the Helsinki Foundation in Warsaw, a rights group that has been urging Polish officials to shed light on the case. "I am afraid that sooner or later the Polish investigation will discontinue the proceedings, as has happened in Lithuania."

The new prosecutor on the case, Waldemar Tyl, stressed the change would have no effect, and called the worries "groundless."

"Nothing will change here," Tyl told The Associated Press.

Tyl also stressed that the other prosecutor, Robert Majewski, would remain on the case. And he said he would keep up efforts to seek information from Washington that could help the case, including obtaining testimony from suspects at Guantanamo Bay who maintain they have been held in Poland.

President Barack Obama is due to visit Poland later this week. It's not clear if the subject will come up in his meetings with Polish leaders.

"I hope this decision is not connected with the visit of President Obama to Poland or the upcoming parliamentary elections in Poland" in the fall, Bodnar said.

Bartlomiej Jankowski, a lawyer for one of the suspects allegedly held in Poland, expressed hope that the investigation will keep on pace and ultimately disclose the full truth about the site.

Jankowski is the Polish lawyer for Abu Zubaydah, a suspected facilitator for al-Qaida who endured enhanced interrogation techniques in Poland and was later taken to a secret prison in Lithuania, according to two former U.S. intelligence officials. He is now back at Guantanamo Bay prison, where he was held for several months in 2003 and 2004.

The Polish president and prime minister in office when the site operated have vehemently denied its existence. But former CIA officials have told the AP that the prison operated from December 2002 until the fall of 2003, and that prisoners were subjected to harsh questioning, waterboarding and in one case a mock execution involving a bitless drill.

United Nations and Council of Europe investigators have also said they have evidence of its existence.

Lawyers for another suspect, Abd al Rahim al-Nashiri filed a case against Poland earlier this month with the European Court of Human Rights in Strasbourg, France. They charged Poland with "active complicity" in the rendition program carried out under then-President George W. Bush.

Bodnar says he hopes the decision to replace the Polish prosecutor will add to the urgency of the case in Strasbourg.


Reporter Adam Goldman in Washington contributed to this story.