A federal grand jury, concluding a year-long investigation, indicted four men yesterday in the killing last July of exiled Iranian diplomat Ali Akbar Tabatabai in Bethesda. But while the long-awaited indictment labeled the killing a conspiracy in which cash payments were involved, it gave no indication of who ordered the killing or who paid for it.

The indictment alleged a conspiracy in which four Northwest Washington men, all of them Americans and two of whom had been identified before, planned and carried out the execution-style slaying of Tabatabai, an outspoken critic of Iranian leader Ayatollah Ruhollah Khomeini.

The indictment charges that Daoud Salahuddin, 30, who was serving as a security guard for the Iranian government representatives here, obtained a U.S. Postal Service vehicle, went to Tabatabai's home disguised as a postman, lured him to the door then shot Tabatabai and fled the country.

It alleges that Ahmed Rauf, 36, helped Salahuddin obtain the mail truck and later disposed of the murder weapon. Salahuddin, also known as David Belfield, and Rauf, also known as Horace A. Butler, had been named earlier in connection with the shooting.

The indictment also charged that Ali Abdul-Mani, 26, also known as Lee Curtis Manning, rented the vehicle used for the escape, and that William Caffee Jr., 34, also known as Kalid, wiped the car clean of fingerprints and abandoned it in the District.

Salahuddin was driven to the Montreal airport after the shooting and boarded a flight to Geneva, the indictment said. He is now believed to be living in Tehran.

Rauf had been held on federal charges until two weeks ago, when he was released on $50,000 bond. He was rearrested after this indictment and was released on $100,000 bond after arraignment in D.C. Superior Court yesterday.

Caffee, 34, is in the D.C. Corrections system. He was convicted of holding up a District of Columbia liquor store after the shooting.

Abdul-Mani is a fugitive and the FBI has offered a reward of $1,000 for information leading to his capture.

Also named, but as an unindicted coconspirator, was Abu Bakr Zaid Sharriff, 27, of Forestville, also known as Al Fletcher Hunter. Shar-[TEXT OMITTED FROM SOURCE]

The indictment followed a year-long effort of a task force of about 50 law enforcement officers, principally the FBI, Bureau of Alcohol, Tobacco and Firearms, and Metropolitan Police Department, to penetrate the Iranian community here in the wake of the Tabatabai killing and large Iranian student demonstrations and nearly 200 arrests here last summer.

The indictment for the first time mentioned an exchange of money in connection with the killing. It said, "coconspirators whose identities are unknown to the grand jury" provided compensation to the four and financial assistance to help Salahuddin escape.

Shortly before the July 22 killing, the indictment says, Salahuddin received "a sum of money for assassinating Ali Akbar Tabatabai from a person whose identity is unknown to the grand jury."

On the day after the shooting, the indictment says, Caffee "telephoned an Iranian national to request that a cash payment be made" to Sharriff. Sources said yesterday that the amount paid to Sharriff was $2,000.

Although some law enforcement officials say they believe the killing was orchestrated through the Iranian government's representatives here, U.S. Attorney Charles F.C. Ruff said last night that he would provide "no guidance" beyond what was contained in the indictment.

Ruff said he expected that during the trial, there would be "some proof" as to how the defendants were able to escape. But the case is considered extremely sensitive because of the delicate relations between the U.S. and Iranian governments. "Even the slimmest guidance is beyond my power to give you," Ruff said.

Not mentioned in the indictment was Tyrone Frazier, 31, the postman who was convicted of accepting $500 for the use of his postal service jeep in the assassination. More serious charges against prosecution.

All four men indicted have been linked to a group known to law enforcement officials as the Islamic Guerrillas in America.

About two dozen persons have been identified as members of the organization, which is based in Washington and espouses violent attacks against the United States government and anyone who opposes Khomeini and his brand of revolutionary Islamic fundamentalism, sources said.

About half of those IGA members identified by law enforcement officials have at one time or another worked at the Iranian interests section. Many of them are former convicts recruited into the organization from prison.

The task force investigating alleged IGA members has conducted thousands of interviews and charged half a dozen alleged members of the group with various firearms violations.

Officials said they found 9-mm shell casings that matched the bullets taken from Tabatabai's body at the Takoma Park home of Musa Abdul Majid, 27, also known as Derrick Pritchett. He is currently a fugitive.

The killing of Tabatabai, which took place during the time that more than 50 Americans were being held hostage in Tehran, was followed by volatile demonstrations in Washington which at one point spilled into a violent clash between pro-Khomeini demonstrators and police.

Nearly 200 demonstrators were arrested, sent to New York for processing but were released.

Tabatabai had served as a press attache in the Iranian embassy here before Shah Mohammed Reza Pahlavi was ousted from power. He became president of the Iran Freedom Foundation, and on at least one occasion said that he feared for his life because of his criticism of Khomeini.