The investigation that led to the conviction on Friday of two men charged with spying for the Socialist republic of Vietnam is continuing, according to government sources closely involved with the case.

"The FBI has a continuing investigation going on in this whole matter," said one Justice Department lawyer, who would not rule out the possibility of more arrests and indictments.

The sources suggested that investigation may focus on others who may have acted as couriers between Vietnamese officials in Paris and espionage suspects in the U.S. Much of the government's case that ended Friday in Alexandria, was built on the role of a double agent for the FBI and CIA named Dung Krall who acted as a courier between former antiwar activist David Truong - one of the convicted men - and Vietnamese officials.

"We don't think Mrs. Krall was Mr. Truong's only courier," said one government prosecutor.

Friends and supporters of Truong - many of whom were also active in the antiwar movement, said yesterday they are not surprised that the government is pursuing its investigation. They describe the probe as a dangerous holdover of the "blind hatreds" generated by the Vietnam war.

"I think this conviction is going to give them all the ammunition they need," said Jacqui Chagnon of Clergy and Laity Concerned, an antiwar church group based in Washington had worked closely with Truong. "McCarthy era, here we come."

Government officials, however, describe the investigation as a normal and expected result of the case, which involved massive use of a Washington FBI counterintelligence unit and other government agencies.

Many of the same issue that the 11-day trial in Alexandria raised are expected to be renewed as lawyers for both Truong and his codefendant, former U.S. Information Agency officer Ronald L. Humphrey, appeal the case to the 4th U.S. Circuit Court of Appeals.

One of the trial's surprises was how little of the government's case against the two men was based on evidence obtained by warrantless electronic-surveillance of the men approved by President Carter. Carter and Attorney General Griffin Bell approved a warrantless telephone wiretap and a microphone being placed in Truong's Washington apartment, as well as secret television taping of Humphrey at USIA headquarters.

Defense attorneys for both Truong and Humphrey have maintained since the court proceedings began that these warrantless surveillances were unconstitutional violations of the Fourth Amendment's right to privacy.

The government has contended, however, that they are legal, and necessary means for the president to protect the United States from foreign intelligence operations. But in the trial prosecutors said they used - only as much of the evidence - as they had to to secure the convictions.

Nonetheless, Assistant U.S. Attorney Frank Dunham, one of the prosecutors, said that the weight of evidence required to prove the espionage counts against Truong and Humphrey was enormous.It required the prosecution to establish that the stolen diplomatic cables in the case would endanger the national defense, though the cables did not mention U.S. armed forces, Dunham said.

"It's too bad that we don't have any intermediate statutes," said Dunham. He was speaking of laws that would be more serious than simple theft of government property laws, but less serious than those of espionage, which carry a sentence of life imprisonment.

"Such a law would require less proof," said Dunham, "but it would also carry less of a penalty. The only people who might suffer would be the press and the public's right to know, but in the context of this case we probably would have been able to get guilty pleas."

Humphrey's lawyers were offered a chance to plea bargain in the week before the trial, it was reported yesterday, possibly for a sentence of 15 years. Warren L. Miller said he refused because "I'm convinced of the guy's innocence."

Lawyers for Truong had privately expressed fears early in the case about the possibility of Humphrey's striking a deal with the government and testifying against their client.

Though no deal was made, there was a conspicuous division in the defenses during the last days of the trial as Humphrey's lawyers tried to prove that their client was the "unwitting dupe" of Truong.

They claimed that Humphrey had no idea Truong was a spy, "if he was a spy," as Warren L. Miller, Humphrey's lawyer, put it.

The government contended, principally on the basis of a tape-recording Humphrey sent to his Vietnamese "common-law wife," that Humphrey knew Truong was a foreign agent, and passed classified documents to him in order to secure her release from Communist Vietnam.

In a simple pidgin English, Humphrey told the woman, whom he called Kim, "(Truong) help very much Kim for you to come out. He has friend in Hanoi . . . He never help nobody else." Another portion of the tape said that Kim should never tell anyone about Truong or speak his name.

Humphrey said the passage was simply the result of having to sneak in simple words for Kim that she could understand him. She speaks very limited English.

Differences in defense strategy may have diminished with the conclusion of the trial and the prospect of appeals, however. Close friends of both Humphrey and Truong have united in attacking the political aspects of the case, which Humphrey once likened to the oppressive political atmosphere described in George Orwell's novel, "1984."

After visiting her brother in jail yesterday, Monique Truong Miller of Los Angeles broke down in tears. "I feel like I've lost my freedom in this country," she said.

Humphrey, meanwhile, has taken to writing verse in his jail cell. One of his poems is called, "The Alexandria Bastille," another he titled "Asian Eyes, or, Never Give Up," and a third is called "Living Without Time."