Eleven of the 12 local U.S. immigration agents, charged with investigating the Iranian demonstrators arrested Sunday, called in sick yesterday after their superiors allegedly ordered them to drop a civil charge against one of the protesters.

Agent John Hambrick, a union spokesman for the agents, denied that the sick calls were part of a protest. "It appears to be a giant coincidence," claimed Hambrick, who was among the 11.

Only the four supervisors and one agent in the investigation unit showed up for work at the local office of the U.S. Immigration and Naturalization Service.

According to informed sources, the agents "became ill" after they filed a charge on Monday against one of the arrested demonstrators, who had refused to divulge his identify, and then saw their superiors order on Tuesday that the charge be dropped.

The demonstrator, one of 173 supporters of Aytollah Ruhollah Khomeini who were arrested, was charged with violating a new regulation requiring all aliens to provide "full and truthful disclosure of all information requested" by INS agents. Failure to comply is punishable by deportation. The regulation was adopted last November as part of a get-tough policy after American hostages were seized in Teheran Nov. 4.

The sources said the agents were strictly enforcing the full disclosure rule on orders decided upon late Sunday night during high-level discussions between acting INS commissioner David Crosland and the Justice Department.

"The [INS] caved in on the very first [case] and that's too bad," one source charged. The Iranian is one of only two arrested protestors who have been released from jail since the demonstration.

A D.C. Superior Court judge ordered the two released on grounds that there was insufficient evidence against them. The remaining demonstrators are still at D.C. Jail for refusing to give their names to police.

The second released Iranian is still being questioned by the INS, officials said.

An INS spokesman denied that any charges were ever brought against the released protesters. The spokesman said when the Iranian was threatened with the charge and a return to jail he divulged his name.

After agents determined that the man was in the United States legally and had violated no terms of his student visa, he was released.

Meanwhile, it was disclosed in U.S. District Court yesterday that federal investigators recovered an empty mail pouch and tag with the address of the former Iranian Embassy and a badge, helmet and clothes similar to those worn by U.S. Postal Service workers from an apartment rented by Horace Butler, 35. He is charged in connection with the Bethesda slaying last week of former Iranian press attache Ali Akbar Tabatabai.

Tabatabai, an outspoken critic of Khomeini's regime, was shot to death at his Bethesda home a week ago Tuesday by a man who came to his door dressed as a postal worker.

Law enforcement authorities have obtained a warrant for Daoud Salahuddin, 29, who they have charged was the triggerman. He is still at large. Salahuddin, who worked at the Iranian Interests Section of the Algerian Consulate in Washington, allegedly drove to Tabatabai's house in a Postal Service jeep which he rented for a $200 down payment from Tyrone Anthony Frazier, a Postal Service employe.

Frazier has told authorities that when he turned the jeep over to Salahuddin at a rendezvous point on Idaho Avenue NW in Washington, he was then driven in a light blue pickup truck to Baltimore and around Maryland for about four hours by a man he has identified as Butler, a self-employed carpenter from Washington. Both Butler and Frazier were arrested shortly after the killing and charged in Maryland as accessories.

At a hearing yesterday before U.S. Magistrate Arthur L. Burnett, FBI agent Henry L. Gaidis testified that officials discovered a set of keys on the floor of the truck allegedly driven by Butler, one of which fit the ignition on the Postal Service jeep. The jeep was recovered by law enforcement officers after it was abandoned about two blocks from Tabatabai's home on the day of the killing.

Burnett sent the case against Butler to a federal grand jury in Baltimore for consideration of an indictment. Butler has been charged with several federal offenses in connection with the shooting, including violation of Tabatabai's civil rights.