The FBI has begun a full investigation of a chapter of the Posse Comitatus, the right-wing group to which tax protester Gordon W. Kahl belonged before his death in a fiery shootout last Friday, officials said yesterday.

"One chapter of the group is under full domestic security investigation, and we are alert to the possibility of violence by various individual members who may be in that or other chapters," Assistant FBI Director Roger Young said.

Young, who would not say which chapter is under investigation, denied published reports that a nationwide probe of the group is under way.

"We are interested in those members who have evidenced a willingness to involve themselves in violent activities, but we have no interest in those who are advocating tax reform because that activity is protected by the First Amendment," he said.

While the FBI can more easily investigate individual members, it is under strict constraints in investigating domestic groups because of guidelines enacted after disclosure in the mid 1970s of abuses in pursuing domestic political groups.

The Posse Comitatus investigation is being conducted under a section of the guidelines aimed largely at domestic terrorist groups, which provides that an investigation can proceed only after a finding that a group is "engaged in an enterprise for the purpose of furthering political or social goals wholly or in part through activities that involve force or violence and a violation of federal criminal laws."

Posse Comitatus, which means "Power of the County," is a militant group of survivalists who challenge all forms of state and federal authority, including all laws and the power to levy taxes. In theory, the group recognizes only local county-level authorities, but in fact, members of the group have also resisted local laws, including traffic and land-zoning regulations.

Federal law enforcement sources said yesterday that many members of the group are heavily armed, that they are involved in paramilitary training, and that the group, which is virulently anti-black and anti-Semitic, has drawn membership from the Minutemen, a right-wing extremist group that went underground in the mid-1960s.

Posse Comitatus was a relatively obscure organization until last February when two federal marshals were shot and killed while attempting to arrest Kahl and others at a roadblock in North Dakota. Kahl, who was a fugitive at the time because of a parole violation in a tax case, had eluded police since then.

Federal sources said yesterday that many Posse members have joined the "Life Science Church," turning over their property and possessions, declaring themselves ministers, and collecting church salaries, to avoid taxes.

One federal agent said members tend to be obsessed with the possibility of a Soviet takeover. "Everywhere they go, they have guns and ammunition, and enough food and water to last them 30 days, in the trunk of their car if necessary, because the Commies could take over any minute," he said.

Membership figures are not known, but leaders claim to have chapters in every state except Hawaii, with as many as 2 million members nationwide. Federal law enforcement officials place the membership at between 3,000 and 10,000 nationwide.

All over the country, members of Posse Comitatus have had confrontations with law enforcement officials, some minor and some more serious. Two years ago three undercover agents from the federal Bureau of Alcohol, Tobacco and Firearms were shot by a member who planned to sell them machine guns and homemade bombs.

One BATF official said that investigations of individual members have been conducted in 26 states, generally involving firearms and explosives violations. He said the heaviest concentration of members appears to be in the north central and the northwest parts of the country.

Agents from the Internal Revenue Service have also been the targets of Posse Comitatus assaults. In 1980, The Washington Post obtained an internal IRS document warning key IRS personnel in New England about possible danger from Posse Comitatus, warning that the agency had "learned that the Posse Comitatus is allegedly planning to lure revenue employees to isolated places and murder them.

"They have prepared a pamphlet which describes a procedure whereby a phony address is used to lure the individual to a particular area and, while he/she is searching for the nonexistent address, the individual is 'eliminated' from a distance," it said.