As authorities stay tight-lipped about details of the man's case, speculation continues to swirl.
The Association for Civil Rights in Israel said it sent a letter to Israel's attorney general about the case on Wednesday, criticizing the censorship and calling for the gag order to be scaled back further.
"What is far more concerning, of course, is the fact that a man was held in detention under heavy secrecy, and nothing was published about the reason for his arrest or the circumstances surrounding his death," wrote Dan Yakir, the association's chief legal counsel.
The letter argues that there is "considerable public interest" in more information about the investigation into the death of Prisoner X.
"Was it really suicide? Was there negligence in the supervision of the detainee? Has any official body taken responsibility? What steps have been taken to prevent the recurrence of similar events in the future?" Yakir asked.
Israeli government officials have made no further comments on the case.