American FBI agents have been accused of "turning a blind eye" to the alleged torture of two US citizens of Pakistani origin by Pakistan's security services.
Relatives of the pair campaigned to secure their freedom
The international pressure group, Human Rights Watch (HRW) said that the brothers were routinely tortured, and that US agents did nothing to stop it.
HRW said that Zain and Kashan Afzal were interrogated on at least six occasions during an eight-month period.
A Pakistan government spokesman made no comment. The FBI denied any wrongdoing.
"Any assertion that the FBI condoned or directed torture is false," the FBI said in a statement.
"Longstanding policy of the FBI does not permit any such interview tactic, nor does the FBI condone, direct, or support the use of torture."
HRW says that the brothers were abducted in August from their home in the southern Pakistani port city of Karachi under suspicion of being sympathetic to militant Islamist groups.
But it says they were released in April without being charged.
HRW says that during their detention they were "routinely tortured" to extract confessions of involvement in terrorist activities.
"It is outrageous that Pakistan abducts people from their homes in the middle of the night and tortures them in secret prisons to extract confessions, all the while ignoring court orders to produce their victims in court," HRW's Asia director Brad Adams said.
"The United States should be condemning this, but instead it either directed this activity or turned a blind eye in the hopes of gaining information in the war on terror."
Instead, HRW says, US personnel threatened the men with being sent to the US detention facility at Guantanamo Bay if they did not confess to involvement in terrorism.
The human rights group says that its information is based on "extensive and separate interviews" with the two brothers since their release and other sources.
The BBC's Aamer Ahmed Khan in Karachi says that the pair have refused to speak to local media since their release, and that the Pakistani government has refused to comment on the case.
HRW says that while the brothers were being detained, their mother and Zain Afzal's wife attempted to lodge an abduction case with the local police.
But the police refused to register the case, informing them that "this was a matter involving the intelligence agencies".