Meretz MK Nitzan Horowitz revealed on Wednesday that he sent a letter in June 2010 to Attorney General Yehuda Weinstein calling for a thorough investigation of an anonymous prisoner who was reportedly being held in solitary confinement.

That prisoner was “Prisoner X,” who was identified on Tuesday by an Australian television report as a Mossad agent named Ben Zygier, an immigrant to Israel from Australia.

Zygier was found hanging by a noose in Ayalon Prison in Ramle in 2010, despite being held in a special cell designed to prevent suicide attempts. The case became known as the “Prisoner X” affair.

On his Facebook page, Horowitz posted a copy of the letter, which he had written after seeing a report about the unnamed prisoner published on Ynet. In the letter, he warned Weinstein that “imprisoning a person in complete isolation and total anonymity is a very grave issue.”

Horowitz said Wednesday that a gag order was placed on the entire incident not long after he sent his letter to the attorney general. Nevertheless, Horowitz alleged that a senior law enforcement official spoke with him at the time, and told him that the issue was under “judicial supervision.”

According to Horowitz, the official also assured him that that there was no “prisoner of darkness” and the entire matter was under control.

“Now it turns out that the reality was different,” Horowitz said, adding that the details of the affair indicate that Zygier died shortly after he sent his letter.

Now, the attorney general must immediately open an investigation into the circumstances of his imprisonment and death, and especially make sure that that are no other ‘prisoners of darkness,’” he added.

In speaking out, Horowitz joined a group of politicians who on Tuesday demanded answers regarding the circumstances surrounding Zygier’s imprisonment and death which had been reported by ABC Australia.

The parliamentarians, Ahmed Tibi of Ra’am-Ta’al, Meretz party chair Zahava Gal-on, and Nachman Shai of the Labor Party, asked Justice Minister Yaakov Neeman for clarifications. Neeman said the matter was not part of his purview, adding that questions about such matters should be out to the minister of public security. Still, said Neeman, “there is no doubt that if these claims are correct, this has to be checked.”

Former Israeli foreign minister Avigdor Liberman panned attempts by the MKs to blow the case wide open, saying they identified with terror groups and were trying to undermine the censor.

“It is first and foremost an attempt to damage Israel’s security,” Liberman told Israel Radio on Wednesday.