Senate Republicans on Friday pulled out of a bipartisan investigation into controversial “war on terror” detentions and interrogations, including tactics widely condemned as torture.

The move by the opposition party dealt a sharp blow to the Senate Intelligence Committee’s efforts to find out exactly what methods were used when and whether they paid off — without prosecuting witnesses or agents thought to have committed abuses.

Senator Kit Bond, the panel’s top Republican, blamed Attorney General Eric Holder’s investigation into alleged CIA abuse of detainees, which he said made it impossible for current or former CIA officials to work with the committee.

“Had Mr Holder honored the pledge made by the president to look forward not backwards, we would still be active participants in the Committee’s review,” Bond said in a statement.

The Justice Department “sent a loud and clear message that previous decisions to decline prosecution mean nothing and old criminal charges can be brought anytime against anyone,” said the Missouri senator.

“Against these odds, what current or former CIA employee would be willing to gamble his freedom by answering the committee’s questions?”

As a result, he said, the committee would be “spinning our wheels” and “cannot complete its review in a reasonable time if witnesses won’t talk to us.”

Democrat Dianne Feinstein, who chairs the Senate Intelligence Committee, declared: “I very much regret” the decision, but vowed to pursue the panel’s efforts.

“That study and investigation is being pursued, additional staff are being hired, and the Committee is continuing the work with all due diligence,” she said.

Feinstein said in late August that Holder’s investigation was poorly timed, even as former Republican vice president Dick Cheney blasted the probe into possible lawbreaking by CIA agents as “an outrageous political act.”

After reading classified accounts of US questioning of detainees in Iraq and Afghanistan, Holder in late August picked Assistant Attorney John Durham, a career prosecutor, to determine whether a full investigation was needed.

Durham will investigate whether CIA officers and contractors broke US law when trying to scare terror suspects detained overseas.

It remains unclear whether Holder would chose to go ahead with prosecutions if Durham’s investigation concludes that US laws were violated.