Human Rights Watch on Sunday called for a thorough and impartial investigation into the death of two Tunisian detainees in suspicious circumstances in separate cases.
"Relatives of both men provided photographs showing marks and bruises on their faces and bodies, and said that authorities had failed to inform them promptly about the deaths," the New York-based rights watchdog said.
A Tunis court had launched preliminary inquiries into the deaths of Sofien Dridi and Qais Berrhouma HRW cited their families' lawyers as saying.
Dridi's family found him dead in a Tunis hospital morgue on September 18, seven days after police arrested him at the airport following his deportation from Switzerland, HRW said.
A spokesperson for the prison to which Dridi was transferred four days after his arrest said in a televised interview that he suffered from diabetes, which caused a fatal heart attack, it said.
The National Guard anti-drug brigade arrested the second man, Berrhouma, on October 5 when witnesses saw plain-clothes men violently beat him in the street.
The next day, his family learned through a relative that his body was at the same hospital.
"The Tunisian authorities should carry out a thorough and impartial investigation of these deaths," HRW said in a statement.
HRW researcher Amna Guellali said: "The credibility of Tunisia's judicial system will take a big hit if it can't explain how and why these two detainees died."
The watchdog has documented three other cases of suspicious deaths in custody: Abdelmajid Ejday in May; Mohamed Ali Snoussi last year; and Walid Denguir in 2013.
The UN special rapporteur on torture, Juan Mendez, last year labelled Tunisia's efforts to eradicate the practice "disappointing" in spite of "very encouraging developments" in other areas of human rights reform.