(CNN) -- Rights groups are calling for the release of the body of a South African freelance photojournalist and investigation into the role Moammar Gadhafi's forces played in his death in the Libyan desert.
The fate of Anton Hammerl, who had been missing since early April, came to light this week when two journalists detained by Libyan forces came forward after their release to say the 41-year-old had been shot six weeks earlier.
In a joint statement late Friday, Human Rights Watch and the Committee to Protect Journalists accused Libya of deliberately withholding information about Hammerl.
"Libya was obliged to take all feasible measures to account for persons reported as missing as the result of fighting and provide their family members any information it had regarding their fate," the statement said.
"Hammerl's family had repeatedly sought information about his whereabouts."
For weeks, there have been conflicting reports about the fate of Hammerl, who was last seen April 5, 2011, covering fighting between Gadhafi's forces and rebels near al-Brega, a key oil town in eastern Libya.
The South African government said it received repeated assurances from the Libya and even Gadhafi that all the journalists were alive and in Libyan custody.
But Libyan government spokesman Musa Ibrahim has said the government was not holding him and had not been able to locate him.
"We never had him with us at any stage," Ibrahim said.
However, CPJ and Human Rights Watch have disputed Ibrahim's account, saying that "credible sources reported that the Libyan government possesses Hammerl's passport, and so was aware of his identity and his fate."
CNN has not been able to independently confirm the rights groups claim.
Hammerl, who holds South African and Austrian citizenship, was initially believed to have been detained along with journalists Clare Morgana Gillis, a freelancer for the Christian Science Monitor, the Atlantic and USA Today; James Foley of GlobalPost and photographer Manuel Varela, who also goes by the name Manu Brabo.
Austria's foreign ministry also said as early as April 25, 2011, Hammerl was alive and that it was negotiating his release with authorities in Tripoli, according to Reporters Without Borders, a nonprofit organization that defends press freedoms and has been working to obtain information about the fate of missing journalists in Libya.
When Gillis, Foley and Varela were released along with British journalist Nigel Chandler, they contacted Hammerl's wife to tell her what happened, according to interviews they gave after their release to GlobalPost and the Atlantic.
Gillis and Foley said in the interviews that they were traveling with Hammerl, following rebel forces toward the frontlines near al-Brega, when they were attacked by Gadhafi's forces.
The two said in the interviews that Hammerl was shot in the abdomen by Gadhafi's forces. They last saw him lying in a pool of blood as they were taken away, according to the interviews.
"The Libyan government chose to remain silent about Hammerl's fate, even though they knew he had been killed," said Mohammed Abdel Dayem, CPJ's Middle East and North Africa program coordinator. "That's not only cruel, it's unlawful."
The Austrian government also criticized Gadhafi's government, saying it was "very disappointed" that Libya did not tell it of Hammerl's death.
"Now we hope they will be cooperative and show us where he is buried so we can bring him to his family for a proper burial," Otto Ditz, Austria's ambassador to South Africa, said through CPJ.
Hammerl's family is taking up the issue of the return of his remains through the South African government, said family friend Bronwyn Friedlander.
Hammerl was a former photographer for the The Saturday Star in Johannesburg. He had gone to cover the fighting in Libya in late March as a freelance photographer.
Since February, the Committee to Protect Journalists has tallied more than 80 attacks on journalists in Libya, including four deaths and 49 detentions.
Among those killed were Oscar nominee Tim Hetherington and acclaimed photojournalist Chris Hondros, who were struck by a rocket-propelled grenade while chronicling the violence in the besieged port city of Misrata.
CNN's Jomana Karadsheh and Chelsea J. Carter contributed to this report.