Two Tunisian men held in secret CIA prisons for more than a year have told a leading human rights organization they were tortured with gruesome and previously unknown techniques.

The men, who were released to Tunisian custody in 2015, described being threatened with placement in an electric chair at a black site prison in Afghanistan in 2002; being beaten with metal batons while their arms were suspended by a bar above their heads; and having their heads pushed into barrels of water.

One of the men, Ridha al-Najjar, was a pivotal detainee for the CIA, which believed him to be a bodyguard for Osama bin Laden. Najjar was the first man taken by the CIA to the black site, which was code-named Cobalt and was where at least one detainee is known to have died. His interrogation became a template for others at the site, according to the CIA inspector general. Najjar said the interrogators forcibly inserted something into his anus.

According to a footnote in the 2014 Senate intelligence committee's investigation into torture, John Brennan, now CIA director, was among the senior CIA officials briefed in the summer of 2002 on the interrogation plan for Najjar. According to the Senate report, the plan included isolation, "'sound disorientation techniques', 'sense of time deprivation', limited light, cold temperatures, and sleep deprivation".

"There was a barrel full of water, and they kept submerging [my head] in the water," the other Tunisian man, Lotfi al-Arabi El Gherissi, told Human Rights Watch, which shared the two men's accounts with the Guardian.

There was a barrel full of water, and they kept submerging [my head] in the water

Lotfi al-Arabi El Gherissi

Najjar said: "They would do this until I couldn't handle it anymore and I was on the verge of completely falling apart."

El Gherissi and Najjar both appear by name in the declassified portions of the Senate torture report, which corroborates their CIA captivity, which began in 2002. It further documents that El Gherissi spent more than a year in the black sites and Najjar spent nearly two.

Both men were subsequently transferred into US military custody at the prison on the outskirts of the massive Bagram Airfield complex, an hour's drive from Kabul. They remained there without charge until June 2015, when the US abruptly transferred them to Tunisia for release. El Gherissi and Najjar now live freely but without work, restitution from the US or even an acknowledgment of the treatment they endured.

"The US should provide these men with adequate compensation and redress," said Human Rights Watch's Laura Pitter. "It is under an obligation to do so under treaties it has ratified and there is still time to act.

"If it doesn't respect its obligations when it comes to its own very serious human rights violations, it undermines US credibility when urging other governments to respect theirs."

The two men's accounts, which were delivered to Human Rights Watch independently of each other, underscore the degree to which the CIA's post-9/11 torture remains obscured from public view, even after the public release of 525 pages of the 6,700-page Senate torture study.

That report, which documented extensive abuse and even death in CIA custody, did not indicate that the agency prepared any makeshift electric chairs, nor that interrogators beat detainees with metal rods.

The report describes extensively the insertion of water or pureed foods into the anuses of detainees, which the CIA denies was sexual assault. Najjar's account of having an unknown object repeatedly placed in his anus has nothing to do with nutrition. He told Human Rights Watch it occurred when interrogators sought to move him from place to place. After the object's insertion, he said, "they carried me upstairs like a package".

Daniel Jones, the former Senate staffer who led its torture investigation, said CIA operatives at the Cobalt black site kept such poor records that it was "impossible for us, or the CIA, to fully determine how many individuals were detained there - let alone everything that happened to them."

Jones noted that that Najjar featured significantly in the CIA's misleading assertion that its so-called "enhanced interrogation techniques" or EITs, led the US to Bin Laden.

[It was] impossible for us, or the CIA, to fully determine how many individuals were detained [at Cobalt]

Daniel Jones, Senate torture investigation leader

"In May 2011, al-Najjar was one of the detainees the CIA cited as having been subjected to the CIA's EITs and then providing information on Abu Ahmad, the UBL assistant who led to UBL," Jones said, using an acronym for Bin Laden.

"However, the Senate found that the information al-Najjar provided was acquired not only prior to the CIA's use of EITs against al-Najjar, but also prior to al-Najjar being transferred to CIA custody."

El Gherissi and Najjar's presentation describing having their heads pushed into barrels of water also indicates that the agency's water tortures went beyond even the widely known technique of waterboarding, in which a restrained detainee has water poured over his nose and mouth to induce a feeling of drowning.

Their accounts of submergence also suggest that a lesser-known technique, "water dousing", wherein interrogators dumped large volumes of cold water on detainees that in some cases obstructed breathing, was both more extensive and more similar to waterboarding than previously understood.

Najjar told Human Rights Watch that in addition to having his head dunked in a barrel of water between interrogators' questions, he was also strapped to a board and lowered by his head, upside down, into the barrel. In a description consistent with other accounts of water dousing, he reported being placed within a tarpaulin full of cold water that zipped closed at the top with minimal space for air.

The CIA has maintained that it only waterboarded three detainees: Khalid Sheikh Mohammed, Abu Zubaydah and Abd al-Rahim al-Nashiri. Earlier accounts of water dousing come from Mohammed and 12 other detainees, none of whom are Najjar or El Gherissi.

In 2015, a lawyer for a detainee subjected to water dousing told the Guardian: "You would laugh the next time you heard the government try to minimize its wrongdoing by drawing a distinction between waterboarding and other forms of water torture."

Former detainees describe unreported CIA torture, in a video released by Human Rights Watch.

A spokesman for the CIA, Ryan Trapani, told Human Rights Watch the agency "reviewed its records and found nothing to support these new claims". The CIA told the Guardian it would not comment further. It has conceded that the earliest years of the black sites, when Najjar and El Gherissi were held, were their period of greatest abuse.

Other aspects of Najjar and El Gherissi's torture correspond with known techniques. They were placed in stress positions, subjected to extensive isolation and sensory deprivation and kept awake for long hours. Both were kept for long periods in diapers, a technique CIA director George Tenet approved in a January 2003 memorandum, with Najjar having no access to toilet facilities, according to the Senate torture report.

Najjar said an interrogator cocked a gun at the back of his head and threatened him with death if he did not talk. Nashiri was also threatened with a gun.

As with other detainees, torture techniques were often used in combination. Najjar said having his hands suspended above his head by a metal bar made it impossible to sleep.

"They hung me up from my hands, and I was standing on the tips of my toes for 24 hours. That lasted for about a month," El Gherissi said. The Senate torture report states that he underwent "at least two 48-hour sessions of sleep deprivation in October 2002".

Within the first month of his captivity at the black sites, according to the Senate torture report, CIA cables described Najjar as "clearly a broken man ... on the verge of a complete breakdown". His torture "became the model" for others at the black site, according to the CIA inspector general.

Human Rights Watch conducted its interviews with Najjar and El Gherissi in August, over a year after their release from Bagram. Their confinement at the airfield detention center for more than a decade prevented them from speaking earlier about their torture - a consequence of judges' support for the US justice department's contention that wartime detentions in Afghanistan were beyond the jurisdiction of federal courts.

I believe one of the primary motivations for the decision to hold our clients at Bagram was to keep their torture secret

Tina Foster, attorney

"Absolutely, I believe one of the primary motivations for the government's decision to hold our clients at Bagram (rather than Guantánamo, for example) was to keep their torture a secret," Tina Foster, an attorney for Najjar and El Gherissi, said in an email.

"Even after President Obama was elected and vowed not to continue the torture program, his administration continued to argue that US courts should not review our client's cases and prevent us from speaking with them. As a result, our clients were held incommunicado and their abuse was effectively concealed for more than a decade."

Both Najjar and El Gherissi said they continued to experience chronic pain as the result of their CIA abuse and insufficient subsequent medical treatment. Najjar said he suffers blood in his stool, liver and kidney problems and more.

"The damage is with my back, I can't sleep on it," El Gherissi said.