Daoud Salahuddin, the international fugitive charged with last month's murder of an Iranian exile leader here, flew to Iran within three days of the shooting, according to information obtained by investigators in the case.

Officials who have been tracing the 29-year-old Salahuddin's escape route believe that on July 22, within hours of the assassination, the suspect took an airline shuttle from National Airport to New York.

There, they say, he boarded a flight for Europe, arriving in Switzerland sometime the next day. By Friday, July 25, sources said, investigators had information that Salahuddin was in Iran.

If the information, which investigators believe to be reliable, is confirmed, there is little chance that Salahuddin will ever be returned to this country to stand trial on charges of murdering former Iranian press attache Ali Tabatabai.

The United States formally severed diplomatic relations with Iran last April, five months after Islamic militants took American diplomats hostage in Tehran. In the absence of diplomatic relations or any extradition treaty, investigators have no reason to believe they can force Iranian authorities to arrest Salahuddin or return him to the United States.

Tabatabai, an outspoken foe of Ayatollah Ruhollah Khomeini, was gunned down on the doorstep of his Bethesda home shortly before noon two weeks ago by an assassin disguised as a postman.

Within two hours of the shooting, Salahuddin had been identified as the key suspect in the case. But while they tried to pass information about the suspect to U.S. airports as quickly as possible, "it wasn't quick enough," one source said.

"He's gone now," the source added.

FBI agents are currently trying to determine which flight Salahuddin took to Europe. They also are checking whether he used the name Salahuddin -- which he has used since he converted to Islam 10 years ago -- or his original name, David Belfield, or a third, possibly Iranian name.

One source said it is probable that Salahuddin used one name while traveling from New York to Europe and another after changing planes en route to Switzerland.

Investigators also are trying to confirm reports that, while in Switzerland, Salahuddin may have received aid from Iranian diplomatic personnel or that he visited an Iranian embassy or consulate.

In a telephone conversation from Bern, the Swiss capital, Iranian ambassador Golam Ali Farivar Pehrani told Washington Post special correspondent Anne Crosman, "I don't know anything about this. I don't know anything about your report. He [Salahuddin] did not come to Switzerland."

Ulrich Hubacher of the Swiss Justice and Police Department told Crosman yesterday that neither his department nor the international police in Switzerland have any record of Salahuddin in their files.

FBI agents here now suspect that Salahuddin had a passport and an airplane ticket ready before Tabatabai was slain.

One acquaintance of Salahuddin said shortly after the slaying that Salahuddin had returned to his sometime residence -- the Islamic House on 16th Street NW -- shortly after Tabatabai was shot. The alleged assassin took a shower and changed there before leaving, the acquaintance said.

Montgomery County police issued a warrant formally charging Salahuddin with murder after two witnesses -- an Iranian who saw the shooting and a U.S. Postal Service employe who has been charged with aiding Salahuddin -- identified the suspect's picture from D.C. police files.

The postal employe, Tyrone Frazier, originally told police that three men kidnaped him and hijacked his postal Jeep shortly before the shooting. After 10 hours of questioning, however, police say, Frazier changed his story and admitted loaning his Jeep to Salahuddin in return for a payment of $200 and a promise of $300 more.

By the time Frazier gave his statement, police had already notified airport security personnel to be on the lookout for Salahuddin. Now, however, they believe this notification came too late.

The suspect's quick departure indicates to some law enforcement officials that Salahuddin, who worked in the Iranian Interests Section of the Algerian Embassy -- the only remaining Iranian diplomatic presence in Washington -- may have had aid in planning the murder and financing his escape.

It is not clear, however, why FBI agents tried to search the office of the Interest Section on July 25 and 26, after Salahuddin apparently had turned up in Iran. Algerian Embassy officials invoked diplomatic immunity and refused to allow the search, leading to a two-day standoff at the offices at 2139 Wisconsin Ave. NW.

Eventually FBI agents gave up their quest after an Algerian diplomat assured a State Department official that Salahuddin was not in the emabssy offices.

No one has taken credit for the assassination, although pro-Khomeini leaders in Washington and Tehran expressed satisfaction at the death of a man they saw as an enemy of Islamic rule in Iran.

Tabatabai's assassination was the first apparently political murder in the Washington area since Chilean exile Orlando Letelier was killed by a bomb on Sheridan Circle NW as he drove to work along Massachusetts Avenue in the fall of 1976.

An eyewitness to the Tabatabai slaying said that the assassin came to the door of the Tabatabai home at 9313 Friars Rd. in Bethesda wearing the uniform of a postman, complete with pith helmet.He told the man who answered the door that he had two special delivery packages for Tabatabai. w

When the press attache came to the door and bent down to look at the packages, the assassin fired three bullets from a 9-mm handgun hidden behind a handful of manila envilopes, Tabatabai, wounded in the abdomen, fell back into the foyer. He died at Suburban Hospital about 40 minutes later.

The assassin then fled in a postal Jeep, which investigators found abandoned two blocks away shortly after the shooting. But not until 4:15 that afternoon did Frazier, the postal carrier to whom the Jeep was assigned, call in to his supervisors with the story that he had been kidnaped.

From the outset, police said later, they were skeptical of Frazier's story.

After 10 hours of questioning the postman capitulated and changed his account, police said.

After Frazier identified a photo of Salahuddin, he also named a local carpenter. Horace Anthony Butler, as the man who had driven him away from his Tuesday morning rendezvous with Salahuddin -- the rendezvous at which he turned over the Jeep to the alleged assassin.

FBI agents arrested Butler the day after the assassination and charged him with conspiring to violate Tabatabai's civil rights. Montgomery County police charged Frazier as an accomplice before the fact of Tabatabai's murder, and charged Salahuddin with the murder.

Frazier and Butler are both being held without bond pending grand jury investigations.