By David Rohde and Jonathan Landay

(Reuters) - Two Tunisian men who spent 12 years in U.S. custody in Afghanistan said CIA interrogators tortured them using previously unreported techniques that included threatening them with a makeshift electric chair and beating them with batons so brutally that they suffered broken bones, Human Rights Watch reported on Monday.

The accounts, which could not be independently confirmed, raised new questions about how prisoners were treated in a former CIA prison in Afghanistan that remains shrouded in secrecy.

Ryan Trapani, a CIA spokesman, said the "CIA reviewed its records and found nothing to support these new claims."

But Daniel Jones, who led a Senate investigation into the CIA detention program, said the accounts given by the two men, Ridha al-Najjar, 51, and Lotfi al-Arabi El Gherissi, 52, were important because so little is known about the "Cobalt" black site, where an Afghan detainee froze to death in 2002.

"The committee found that the COBALT detention site kept so few written records that it was impossible for the Senate, or the CIA, to determine how many individuals were detained there," Jones said. "And what exactly was done to those detained."

The two men had not publicly described their treatment before, Human Rights Watch said. Interrogators accused Najjar of being a bodyguard of Osama bin Laden and Gherissi of being an al Qaeda member, but both were returned to Tunisia in 2015 and released. 

Gherissi said his guards put him in the makeshift electric chair, which he said had plugs for prisoners' fingers with wires attached and a metal cap for a prisoner's head. The former detainees said they were not electrocuted and the chair was attached to a wall pipe.

An independent medical expert hired by Human Rights Watch said that X-rays of Najjar showed his ankle had been broken and had not healed properly. Gherissi was missing two teeth.

A September 2002 CIA cable described Najjar as a "clearly broken man" when he was held at Cobalt who was "on the verge of a complete breakdown."

Despite that assessment, the CIA continued to torture Najjar, according to Laura Pitter, the Human Rights Watch investigator who interviewed the former detainees earlier this year in Tunisia. Pitter called for the United States to pay both men compensation.

Both former prisoners now live with family members, are impoverished, and told Pitter they were struggling to recover from the abuse they suffered in U.S custody.

(This story corrects paragraph 8 to say that Gherissi, not Najjar, was missing two teeth, clarifies in paragraph 6 that Najjar was accused of being a bin Laden bodyguard, not al Qaeda member, and clarifies in headline and paragraphs 1 and 7 that chair was makeshift, not mock)

(Reporting by David Rohde in New York and Jonathan Landay in Washington; Editing by John Walcott and Peter Cooney)