When the Carter administration wanted to slip U.S. Army officers into Iran to help prepare for an attempt to rescue the American hostages held there in 1980, the Army created its own Intelligence Support Activity ISA to carry out this covert mission.

Three years later, after Congress thought it had closed down, the ISA still exists and is growing, shrouded in secrecy. Its critics, including former CIA director Stansfield Turner and some members of the intelligence oversight committees in Congress, argue that it is unnecessary and potentially troublesome for the Army to have its own intelligence arm.

A four-star general, who spoke about the ISA's origins on the condition that he not be identified, said, "We had some assets that the CIA needed for humint," referring to human intelligence. "So we made them available for the hostage rescue operation.

"After that raid was aborted, we decided to keep the organization intact. Casey is all for it," he said, referring to CIA Director William J. Casey.

Other sources said the ISA began in 1980 with a budget of $2 million, which has tripled since then, although it still is small compared with those of the CIA or the Pentagon's Defense Intelligence Agency. They said that the ISA is run by Col. Jerry M. King out of a building in the Arlington Hall military complex in Arlington.

Army Secretary John O. Marsh Jr.'s desk calendars, which were obtained by The Washington Post, show that he discussed the ISA with Maj. Gen. William E. Odom, assistant chief of staff for intelligence, and other senior Army officers in at least nine separate meetings last year.

An Army specialist stationed at Fort Bragg, N.C., said in an interview that he is on call for special assignments by the ISA. Congressional critics said that the ISA's real size and budget are difficult to determine because it can call on such specialists stationed throughout the world after selecting them by computer.

The Department of the Army has refused to discuss this or other aspects of the ISA. However, in response to inquiries from both congressional intelligence committees and the press, the Army denied that the ISA is operating in Central America, as was alleged recently by numerous callers to the congressional committees.

Asked about the continued existence and growth of the ISA long after the aborted hostage rescue mission, a member of the Senate Select Committee on Intelligence said, "I thought we killed that snake."

Turner, who served as President Carter's CIA director, said "It's not a good idea" for the Army have its own intelligence agency.

"First, I don't think that the military is very adept at this kind of clandestine, covert activity," Turner said. "Second, it's a bad idea to set up a competition in this activity."

If both the Army and the CIA are operating agents and doing other covert work, he said, "they're likely to run into each other in back alleys overseas. They will be bidding against each other for information and agents. There's not room for two agencies to compete for clandestine resources."

"The military can't have all the resources they think they need under their own control," Turner said. "They have to share communications satellites and intelligence networks because the country can't afford to let everybody have his own intelligence system.

"The military has got to understand that there has to be cooperation. Satellites are too costly and spies are too costly for everybody to have his own intelligence operation. The military trying to get out of reach of the decision-makers isn't good for the country. It's part of a general mood of the military trying to get its own of everything."

Retired Lt. Col. James G. (Bo) Gritz told the House Foreign Affairs subcommittee on Asian affairs last March that a special Army outfit he called "the activity," which other sources said was a reference to the ISA, was interested in helping him with an operation named Grand Eagle to rescue American servicemen who he said he believes are being held captive in Laos.

According to Gritz' testimony, the ISA withheld its support after becoming embroiled in jurisdictional disputes within the intelligence community.

"Jan. 4, 1982, I received a telephone call at my home in California," Gritz testified. "The chief of that activity on the phone said to me: 'Bo, I have been ordered to put Grand Eagle back on the shelf as if it never existed. There is something here that we cannot see but we can certainly feel. There are still too many people that do no want to see POWs prisoners of war returned.'

"The activity was a field unit and would have put an American across into Laos to verify, using various recording means, the presence of Americans thought to be at specified locations" in Laos, Gritz told the subcommittee.

Army officials later denied that the ISA sponsored any of the Gritz forays into Laos.