Although Libya's diplomats are now gone from Washington, expelled by the Reagan administration for supportiong "international terrorism," concern over Libyan activities continues here and abroad. Among the developments:

The FBI is investigating possible links between a former U.S. Green Beret charged with shooting a Libyan student in Colorado last October and two fugitive former CIA officials who are wanted on federal charges of helping the Libyan ruler, Col. Muammar Qaddafi, recruit, arm and train terrorists, according to law enforcement officials.

U.S. officials say visa applications by Libyans to visit this country will now carry what is called a mandatory security advisory opinion, which means they will be screened not just by the State Department but by other agencies, including law enforcement and intelligence branches.

In England, another Libyan dissident was found murdered last week and authorities believe the slaying may be linked to a Qaddafi-inspired campaign to silence critics living aborad. Last year, officials here say, 11 expatriate critics of Qaddafi were murdered in England, Greece, Italy, Lebanon and Malta.

The small West African state of Liberia has also announced it is ousting the local Libyan "People's Bureau," as the Libyans call their embassies. Within the past year, Gambia, Senegal, Niger, Morocco, Kenya, Iraq and Saudi Arabia have either broken diplomatic relations with Libya over charges of internal interference or foreign policy disputes, or have refused to accept People's Bureaus as substitutes for traditional embassies.

The former U.S. Army Green Beret charged in the Colorado shooting is Eugene A. Tafoya. According to the arrest affidavit, Tafoya is "suspected of being a mercenary who is avilable to be hired and paid to kill human beings." According to court records, Libyan money and literature about Qaddafi were among the items found in a search of his home.

From the time Tafoya was arrested in April, the FBI has been investigating possible links to Libyan authorities. But what may now increase interest in the case is the question of whether Tafoya is one of the former U.S. military specialists allegedly recruited for Qaddafi by the two former CIA employes, Frank J. Terpil and Edwin P. Wilson. FBI sources say the bureau is looking especially into possible links between Tafoya and Wilson, who previously lived on a farm in Upperville, Va.

If there is a link, it could suggest that Americans may have been involved in other murders or attempts around the world in Qaddafi's behalf, a possibility that was raised in an article by investigative reporters from The Boston Glove that appeard before Tafoya was arrested.

Terpil and Wilson were indicted by a federal grand jury last year but are believed to have fled the country. Officials here believe that, if the charges are correct, the familiarity of the two fugitives with CIA and military techniques ha undoubtedly added a level of sophistication to Libyan destabilization tactics.

Though the Colorado shooting was a factor in the decision to expel the Libyans from Washington, several State Department officials say privately that the overriding reason behind the order was political. It was this administration's desire to make what one official called "a loud public statement that there will be no business as usual Libya decides to play by the rules of international conduct."

On NBC-TV's "meet the press," program Sunday, Defense Secretary Caspar W. Weinberger seemed to take issue with this, linking the decision primarily to "increasing the safety of people in the United States. The Libyan embassies, or People's Bureaus, are really almost assasination headquarters and what we need to do is get people of that kind . . . out of the country," he said.

Nevertheless, in the eyes of many officials, Libya is breaking the international rules by intervening militarily with its Soviet-equipped forces in Chad in northern Africa, by supporting revolutionary and terrorist movements from the Philippines to Northern Ireland, and by creating instability in Africa and the Middle East.

The New York Daily News reported this week that a secret administration plan existed to topple Qaddafi, using Arab states friendly to this country. The State Department has denied any such plan exists, but it is clear that the new administration views Qaddafi as "a menace that we would like to see disappear," as one official said yesterday. However, he added, "there is a long step between saying that and what our policy is," meaning he was not confirming any plan to actually unseat the controversial Libyan ruler.

Administration officials have declined to go beyond the general charges of misconduct by Libya in their public statements. Libya, in turn, has denied charges that it is engaged in terrorism, and has challenged the United States to provide proof.

Privately, government officials who asked not to be identified say the decision to expel Qaddafi's representatives here evolved over the past year.

Last spring, sources say there was a surge of telephone calls to the government from some of the 4,000 Libyan students here claiming they were being harased and threatened by pro-Qaddafi student leaders. This was followed by the wave of killings in Europe and the shooting in Colorado last fall. And in December came the invasion of Chad, which immediately aroused the ire of the incoming Reagan administration, which was formulating a foreign policy based on fighting terrorism and revolution by what it views as Soviet surrogates.

Though there have been hints of Libyan "death squads" operating here, the Colorado case is the only one that has emerged publicly. The FBI, however, is said to be keeping watch on pro-Qaddafi Libyan activist student leaders.

A key remaining question is what will happen to the $12 billion-a-year oil trade between the United States and Libya, which provides about 10 percent of U.S. imports, and to the 2,000 or so Americans who work in Libya.

Qaddafi has repeatedly said those Americans are safe, though he added a mysterious "until the situation deteriorates" in a recent TV interview. The State Department has advised U.S. firms to begin an orderly reduction of their personnel in Libya but this does not appear to have had many takers.

Officials here acknowledge, however, that the U.S. workers are crucial to Libyan oil production, that there is a scarcity of skilled workers from other nations to replace them, and that an exit visa is needed to get out of Libya. Thus, this combination of circumstances could conceivably make things difficult for American firms or employes who wanted to follow State's advice.