Ziad Itani, a stage actor from Beirut, was arrested in November and released four months later. The charges were dropped in the widely publicized case in which details of the allegations were leaked to the media.
After he was released, it became apparent that Itani fell victim to a plot hatched by Major Susan Hajj Hobeiche, the former head of the Lebanese Anti-Cybercrime and Intellectual Property Bureau who was subsequently arrested.
Collaborating with Israel, which is formally at war with Lebanon, can be punishable by death.
The watchdog said Itani's case was worrisome because of the combination of tactics used to pressure him. They included being held incommunicado, torture, forcing him into a confession under duress and threats of rape and physical violence to his family, the group said.
The combination is "very disturbing," said Bassam Khawaja, a HRW Lebanon researcher. "Even more disturbing is that no one has taken the time to investigate."
Lebanon's State Prosecutor Judge Samir Hammoud said he has received no formal complaints, either from Itani or the judge that carried out the investigation.
Lebanon has passed a new law criminalizing torture in November, with procedures to investigate allegations.
Khawaja said Itani's case is a test for the authorities' readiness to implement that law. Allegations of torture are common in Lebanon, yet there have been no reports in recent years of such cases going to trial.
There were no signs the new procedures were followed, Khawaja added.
Itani made his allegations to the military court's investigative judge nearly a month after he was detained, which should have triggered an investigation.
Even though the judge noted the allegations and ordered "an examination," the judge's final report, reviewed by HRW, made no mention of torture or any call for an investigation.
"We have written to both the prosecutor and the military court with the allegations. They have an obligation under both international law and Lebanese law to investigate these allegations. Saying 'I haven't received a complaint' is unacceptable," Khawaja said.
Itani tweeted on Monday that he intends to fight on. "This is the start of the road. I know I am facing political, security and media systems and electronic armies that can do anything to me. But I hope they excuse me because I will continue to my last breath."
Upon his release, Itani, who hails from a prominent Sunni family, was received at home with a hero's welcome. Prime Minister Saad Hariri, a Sunni, hugged Itani after his release from prison, calling his arrest a "mistake."
The security officer who built the case against Itani—the former head of the anti-cybercrime unit—and a hacker have been charged with falsely accusing Itani, and could each face up to ten years in prison.