Two of America’s leading human rights organisations have written to the US attorney general, Eric Holder, calling on him to appoint a special prosecutor to investigate crimes committed under the CIA’s interrogation and detention programme in the wake of 9/11.

The American Civil Liberties Union and Human Rights Watch issued the call as a joint response to the recently published report from the Senate intelligence committee that revealed the brutal extent of the CIA’s use of torture in secret prisons across the world. The groups say the agency’s rendition and treatment of terror suspects amounted to “a vast criminal conspiracy, under color of law, to commit torture and other serious crimes”.

The lack of a full criminal investigation, the letter warns, “would contribute to the notion that torture remains a permissible policy option for future administrations; undermine the ability of the US to advocate for human rights abroad; and compromise Americans’ faith in the rule of law at home”.

President Obama has repeatedly declined to commission a federal review into the CIA’s torture programme, saying the country must look forward rather than back. A limited inquiry was permitted into the CIA’s destroying of video footage of waterboarding sessions, but no charges ever flowed from it.

The ACLU and Human Rights Watch say the publication of the 524-page executive summary of the Senate torture report has put the need for a special prosecutor back on the table.

In their letter, they say the report contained significant new information that underlined “the depravity of the tactics and immensity of the enterprise ... When done as part of a deliberate, coordinated government program, the crimes are more shocking and far more corrosive to US democracy.”

In making the argument for an independent investigation, the two organisations can point to precedent. In 2003, Patrick Fitzgerald was appointed by the Department of Justice (DOJ) to look into the leaking of the identity of a CIA agent, Valerie Plame.

In the past, the Obama administration has argued against pursuing criminal prosecutions of those responsible for the CIA torture programme on grounds that they were operating in “good faith” within the legal parameters set by the Office of Legal Counsel at the time. But the letter to Holder argues that the new material contained in the Senate report reveals the perpetrators knew very well that the interrogation techniques were illegal and sought reassurance from the DOJ that they would not be prosecuted.

As a result, the correspondents say, “the argument of good faith reliance on counsel appears inapplicable to some of the officials who were involved in conceptualizing, ordering, and executing these crimes”.

The ACLU and Human Rights Watch letter formalises a chorus of demands for criminal prosecutions that broke out after the publication of the Senate torture report. Advocates for charges to be brought included the UN special rapporteur on the promotion of human rights, Ben Emmerson, and lawyers at the Center for Constitutional Rights.