Brad Adams, the group's Asia director, said the lawyer's disappearance "reflects glaring problems of state-sponsored abuses in Thailand".
"Now is the time to show concrete progress in the investigation and to bring the perpetrators to justice," he said.
In January Abhisit Vejjajiva, the newly-installed Thai prime minister, eager to show his commitment to human rights and the rule of law, ordered the police and justice officials to speed up investigations into the case.
Adams urged the Thai prime minister to show "political courage" in handling Somchai's case "to end once and for all the enforced disappearances by the security forces".
"Under Thai law, if you don't have a body or a piece of bone to prove a person is dead, it's difficult to charge anyone with murder"
Angkhana Neelaphaijit, missing lawyer's widow
"Solving this case will give hope to many, including in the south, that powerful people can be held accountable."
Resentment against human rights abuses by Thai authorities is among the factors fuelling an increasingly brutal insurgency in southern Thailand, where separatist militants have carried out a string of deadly attacks on civilians and security forces in recent years.
Somchai's widow, Angkhana Neelaphaijit, who is also a human rights activist, said she is still hoping for a clue that might bring her husband's killers to justice.
"Under Thai law, if you don't have a body or a piece of bone to prove a person is dead, it's difficult to charge anyone with murder," she told Al Jazeera.
According to Human Rights Watch four Thai prime ministers in the past five years – Thaksin Shinawatra, General Surayud Chulanont, Samak Sundaravej and Somchai Wongsawat – have acknowledged that police and government officials were involved in Somchai’s abduction and killing.
Despite that no perpetrators were brought to justice, raising questions as to who ordered the abduction and presumed murder, and who was involved in the obstruction of justice.
In 2004 five police officers were arrested in connection with the case and charged with coercion and robbery, but none were charged with the more serious crimes.
One of them was sentenced to three years in jail for assaulting Somchai, but he too, has since gone missing.