A Washington cabdriver who assembled an arsenal in his suburban Maryland apartment and has since fled the country is believed to have furnished the ammunition and perhaps the weapon used in the assassination last July of exiled Iranian leader Ali Akbar Tabatabai, federal prosecutors said yesterday.

The disclosure, contained in a four-page memorandum submitted by prosecutors in federal court in Alexandria, brings to four the number of suspects implicated in the widely publicized slaying, which authorities described as a carefully plotted attempt to silence a leading critic of Ayatollah Khomeini.

According to the memo, FBI agents searching the Takoma Park apartment of Musa Abdul Majid, also known as Derrick Pritchett, shortly after Tabatabai was gunned down found 38 spent 9-mm Luger pistol cartridges. FBI tests later revealed the cartridges were fired from the same gun as two bullet shells found at the Tabatabai murder scene last July 22. Agents also seized 80 boxes of ammunition, as well as a pistol, a rifle and a shotgun at Majid's apartment at 7600 Maple Ave., the memo said.

Majid was described in court yesterday as "a very close friend" of Tabatabai's accused assassin, Daoud Salahuddin, who apparently fled the country within 24 hours after the slaying and is believed to be in Iran. The FBI memo said that ammunition purchased by Majid was "in all likelihood . . . used to kill Tabatabai."

Majid, 26 a self-employed Washington cab driver, failed to appear yesterday before U.S. District Court Judge Oren R. Lewis for sentencing on a Dec. 3 conviction growing out of fraudulent weapons purchases in Virginia. Assistant U.S. Attorney Nash W. Scott told lewis officials believe Majid has left the country, and Lewis said he would sentence Majid as a fugitive. Lewis then sentenced Majid to consecutive three-year prison terms on each of three counts.

The unusual four-page sentencing memo from prosecutors ties Majid for the first time to what authorities have said was a loosely organized group of black American Muslims who embraced the Islamic beliefs of Ayatollah Ruhollah Khomeini, and were violently opposed to his critics.

Tabatabai was shot at point blank range on the doorstep of his Bethesda home as he accepted delivery of two packages from someone posing as a postal carrier. Authorities later said the bogus mailman was Salahuddin, who was also known as David Belfield.

Salahuddin and Majid previously worked together as security men at the Iranian Interest Section of the Algerian Embassy in Washington, according to the court papers made public yesterday. "Salahuddin was employed there up until the time of the assassination of Tabatabai. Majid was fired shortly thereafter," the papers said.

"Majid is associated with a group of individuals who have demonstrated a commitment to illegal and terrorist activities. During the past year . . . this group has been acquiring and stockpiling weapons through fraudulent means for no apparent purpose," the papers continued.

Majid was convicted of buying five handguns and one box of 50 9-millimeter shells from Clark's Gun Shop, Inc., in Warrenton, 50 miles west of Washington, using a non-existent address. Federal officials said they believe one of the handguns, a military-style Walther P-38, may have been the Tabatabai murder weapon.

Another member of the Majid group, Ali Abdulbar, also known as Morris Haywood Smith, also has been convicted in federal court in Alexandria of buying weapons under false pretenses. He is scheduled to be sentenced on Jan. 9.

Two other men have been charged in connection with the shooting of Tabatabai. Horace Butler, a local carpenter, is being held on $50,000 bond on District of Columbia charges that he conspired to commit a murder outside of the District. Tyrone Frazier, a U.S. mailman, faces a federal bribery charge alleging that he allowed his mail van to be used in the shooting incident. He is free on bond.

Law enforcement officials said from the earliest moments of the murder investigation that the motive for the shooting was political. Tabatabai, a balding career civil servant who worked for more than a quarter of a century under the later-deposed Shah, had emerged in his Washington exile as a leading anti-Khomeini crusader.

The former press attache at the Iranian Embassy, Tabatabai often acknowledged that his outspokenness placed him in danger, friends said at the time of his death.

At the same time, a small group of black American Muslims here were ferverently advocating Khomeini's brand of revolutionary Islamic doctrines, according to authorities. Many of them, including Salahuddin, and apparently others of the group federal officials have linked to him, attended Islamic discussion sessions led by Ali Agah, then the head of the Iranian Embassy. Agah was expelled along with his staff in April by President Carter.

The number of American blacks in the pro-Khomeini group is believed to number several dozen. Bahram Nahidian, a Georgetown carpet merchant who has met with the group, has acknowledged knowing Salahuddin, but denied that he was close to him.

Supporters and opponents of Khomeini have clashed in the streets of Washington for several years, but never with such deadly violence as in the shooting of Tabatabai, officials have said.