Yemeni officials have failed for the fourth time to produce at a court hearing an American citizen whose lawyers consider him a disappeared person.

Sharif Mobley, a 30-year-old from New Jersey facing murder charges after initially being suspected of terrorism, was due in court in Sanaa on Wednesday. Instead, judge Abdelwali al-Shaabani heard that Yemen’s attorney general is unsure where Mobley now is.

It was left to a representative for Mobley’s alleged victim to tell the court he had heard from a prison guard that Mobley is now in the custody of the Specialized Criminal Court, a secretive national security court that has been criticized for abuses by human rights groups.

Mobley has not been seen by his lawyers in six months. Left with far more questions than answers, they say they do not know if Shabaani, who has scheduled another court hearing for 10 September, will be able to press on with Mobley’s trial. They suspect the US of involvement with Mobley’s continued disappearance.

Katie Taylor, an investigator with the international rights group Reprieve, which is aiding in Mobley’s representation, said she was unsure if the transfer “represents a shift” in Yemen’s posture toward Mobley or is commensurate with months of denial of legal access.

Last seen by his lawyers on 27 February, Mobley is charged with killing a guard in 2010 in what Yemeni officials consider an escape attempt. Weeks earlier, Mobley, who had come to Yemen to study Islam after what his friends have called his increasing radicalization, was snatched off the street and shot in the leg by local security officials.

During his convalescence, representatives of the FBI and the Department of Defense questioned him about potential terrorism connections, though he ultimately faced no terrorism charges.

US diplomats in Yemen have taken an ambiguous approach to Mobley, a rare American permitted to stand trial in a foreign court, let alone one singled out as abusive. In July, an embassy official, William Lesh, told Reprieve that diplomatic staff had been in touch with Mobley, who asked them to tell his family he is in good health. But the embassy will not disclose where Mobley is, nor would it facilitate lawyers’ access to him.

Lesh refused to answer questions, emailing the Guardian last month: “We cannot comment, we must respect the rights to privacy for all American citizens.”

The Specialized Criminal Court is infamous among human rights observers. The Yemeni government has used it to prosecute journalists who have criticized the regime’s fight against al-Qaida. Freedom House condemned it for “target[ing] both the government’s political opponents and journalists.” Human Rights Watch has written that the court “fail[s] to guarantee defendants’ basic rights to due process” and has called for its abolition.

A Yemeni diplomat in Washington, Mohammed Albasha, said he could not answer the Guardian’s questions about Mobley by press time and requested them in writing.

Taylor said she suspected the Obama administration of involvement in Mobley’s bizarre vanishing from legal view.

“His disappearance this year came on the eve of presenting at trial evidence related to his original abduction in 2010, so it’s awfully suspicious that he’s missing again and the US seems to be offering no aid whatsoever,” she said.