Federal law-enforcement officials say an investigation that uncovered an alleged conspiracy to illegally export U.S. military wares to Iran began when a Customs inspector spotted a folder labeled "Iran 1981-1982" inside the briefcase of a Pakistani businessman arriving at Dulles International Airport.

The business executive, Saeed Zakaria, who maintained an office at 17th and K streets NW, pleaded guilty earlier this year to violating a U.S. export law in connection with the case. Last week a federal grand jury in Alexandria indicted a Canadian munitions firm and two of its officials, charging they schemed with Zakaria to sell M60 tank parts to Iran using a false shipping certificate that would show Pakistan, not Iran, as the destination.

The alleged deal was never completed because Pakistani officials refused to issue the document, according to federal officials.

Neither the company officials, Morris P. Levy and Al Raskin, nor a representative of the Toronto-based firm, Levy Auto Parts of Canada, appeared yesterday at a scheduled arraignment in Alexandria. Arrest warrants have been issued for Levy and Raskin and Assistant U.S. Attorney Joseph Aronica said the government may seek to extradite the two men.

Zakaria, 63, a short, portly man with an accent born of a British empire education, was among the passengers shuffling through Customs at Dulles last March 25 after their arrival from London aboard Pan American Flight 107.

When Zakaria reached the counter, inspector Arthur McAteer asked to see the contents of the executive's black attache case. Inside, McAteer discovered a folder with a clear plastic cover and the words "Iran 1981-1982." And inside the folder, he saw numerous copies of Telexes and documents listing military equipment and price quotations. McAteer asked Zakaria to step out of the line.

"It was a blind hit," said a Customs official who asked to remain unidentified. The official said McAteer was among Customs employes who have been specially trained as part of Operation Exodus, a Reagan administration campaign to clamp down on the unauthorized export of U.S. arms and high technology abroad.

"It was blind--but it never would have been made without the training," said the official.

McAteer radioed for assistance and was soon joined by an investigator who looked through the folder, read Zakaria his rights and started to ask questions.

Federal agents say the resulting investigation brought to light an alleged conspiracy reaching back to April 1981 and spelled out in incriminating detail in the cables, which allegedly were sent among the defendants and a Col. Dehghan, an official of the Iranian defense ministry in Tehran.

The export of items included on a U.S. Munitions List maintained by the State Department requires that would-be exporters apply for a special license and provide a certificate showing the destination of the items. The shipment of American-made M60 tank parts to Iran is illegal.

According to the Alexandria indictment, Levy and Raskin agreed to obtain the tank parts from an unidentified source in the United States and Zakaria was to get the false documents from Pakistan.

On April 7, 1981, the indictment charged, Zakaria, in the role of broker, sent a Telex to Raskin telling him the Iranian government was interested in buying American tank engines for cash. Morris Levy telephoned Zakaria, the indictment alleged, and criticized him for naming Iran as the potential buyer.

Levy ordered Zakaria "never to list Iran as the customer again but that all future correspondence should reflect Pakistan as the buyer," the indictment said.

Zakaria, now more discreet, subsequently sent a list of price quotations to the Iranian government through Iran's ambassador to Pakistan, the grand jury charged.

The contract never went through, apparently because Zakaria failed to get the necessary documents from Pakistan. Last June 11, as the result of a plea-bargaining agreement, District Judge Richard L. Williams gave Zakaria a suspended one-year prison term for aiding and abetting efforts to obtain an export license illegally. Zakaria is expected to testify at Levy's trial.