Iranian militants holding U.S. hostages in Tehran once staged a mock execution of their captives, herding a group into the U.S. Embassy courtyard, lining them up against a wall and making clicking sounds with their rifles, according to reliable sources here.

The episode in the courtyard is said to have been described to high-level State Department officials recently by Ricard I. Queen, the 28-year-old embassy vice consul who was freed by the Iranians earlier this month because of a medical condition that later was diagnosed as multiple sclerosis.

Precise details on what happened and when are sketchy and closely held. A number of senior government officials and congressional figures claim not to know about the incident or decline to comment.

One source close to the situation declined to describe the episode as a mock execution. Rather, he said, it was more like a scare tactic, an attempt on the part of the captors to give the hostages reason to worry about being executed at some point. But there was a general feeling among some of the hostages that "this was it," that they were going to be killed, he added.

Sources say about a dozen hostages were involved in the episode and that Queen was among them.

According to one account, the incident took place shortly after a U.S. efffort to rescue the hostages failed April 24. But another government source contradicts that, saying that it happened before the rescue attempt was aborted in the Iranian desert.

Either way, the event, as described by some top officials, suggests that the conditions of captivity are at least on occasion more severe than is generally known.

During a news conference at the State Department earlier this week, Queen said the harshest period during his 250 days in captivity was a span of two to four months when he was kept in a basement room with no windows.

Yesterday, doctors at Georgetown University Hospital completed medical checks on the young diplomat, and Queen then flew to his parents' home in Maine.

He was greeted on arrival by a crowd of well-wishers, including Gov. Joseph Brennan.