The claim came even as Poland's leaders continued to vigorously deny any involvement.
Marc Garlasco, a senior military analyst with the rights organization, told Poland's Gazeta Wyborcza that Human Rights Watch had documents corroborating its case about Poland, and showing Romania was a transit point for moving prisoners.
"Poland was the main base of interrogating prisoners and Romania was more of a hub," Garlasco told the newspaper in an interview in Geneva, Switzerland. "This is what our sources from the CIA tell us and what is shown from the documents we gathered."
This week, Poland's outgoing President Aleksander Kwasniewski stressed that "there are no such prisons or such prisoners on Polish territory." He went further on Nov. 28, saying "there never have been" such jails.
The Council of Europe has ordered Swiss lawmaker Dick Marty to investigate the matter.
On Thursday, Prime Minister Kazimierz Marcinkiewicz said Poland will open its doors to Marty's investigation and added that his country was "ready to show everything that moves in Poland to guarantee that there are no prisons or such places in Poland."
Still, Garlasco told the newspaper that an operation "on such a scale could not have happened without the knowledge of the Polish authorities. There are people who took part in it, there are flight records."
Garlasco told the newspaper about 25 important terror suspects were interrogated near a former military airport in Szymany, in northern Poland, and in another much larger facility in the south of the country.
He did not show the newspaper the documents that he said were in his organization's possession, but said all relevant material has been passed to Marty.
"We have certain documents, leads, traces to be checked, but it's too early to reveal them," Garlasco said.