A high-ranking FBI official, who reportedly is a target of a federal probe into FBI-committed burglaries, has charged publicly that the investigation is a "vendetta" inspired by Justice Department personnel hostile to the FBI.

Assistant Director Andrew J. Decker said, "It is quite difficult for us to understand the necessity for this prolonged investigation unless we reach the inescapable conclusion that a small segment of Justice Department employees is engaging in a vendetta spawned by smoldering hostility for the FBI."

Decker made the remark in a speech last Thursday at the University of Florida law school in Gainesville, Fla. Although he spoke without text, his statements have been pieced together from notes of persons who heard the speech.

He was referring to the Justice Department's year-long probe into the FBI's use of burglaries, mail openings and wiretapping to obtain information about fugitive members of the Weather Underground in New York in the early 1970s.

Last Thursday - the same day Decker spoke - John J. Kearney, a retired former supervisor in the FBI's New York office, was indicted on wiretapping and mail-opening charges arising from the investigation.

Indictments also are expected against other middle-level supervisory personnel. They are part of a strategy, recommended to Attorney General Griffin B. Bell by Justice Department officials, of using these middle-rank targets as a stepping stone to possible further prosecution of several present and former top FBI executives.

In his Florida speech, Decker acknowledged that "yours truly" has been mentioned in speculation about possible indictments. Since 1974, Decker has been one of the 13 assistant directors at FBI headquarters in Washington. He now heads the records management division.

He admitted to The Washington Post yesterday that he had made a speech in Florida last week, but said he could not comment further.

Justice Department sources said last night that Decker since has given a copy of the speech, apparently prepared from a tape recording, to Bell.

The speech was billed as a discussion of the grand jury process, and Decker used that subject as a springboard for detailing his contention that the grand jury process has been abused and used unfairly in the investigation of FBI involvement in the burglaries.

The FBI, he noted, had been searching for "violent terrorists engaged in a devastating nationwide bombing campaign." It was in the interests of the country to apprehend them quickly, he added, "so the agents, within the scope of their duties and at the instructions of their superiors and without any motive of personal gain, stuck out their necks."

"Should we now decapitate them?" he asked.

In addition, Decker contended, the FBI had engaged in "conduct which was deemed proper at the time and which was approved by many present anf former government officials in positions of authority over the FBI."

He pointed out that in January former Attorney General Edward H. Levi had declined to prosecute members of the Central Intelligence Agency involved in opening mail from China."It would be unfair, hypocritical and perhaps not possible to prosecute persons" engaged in a similar kind of counterintelligence investigation, Decker said.

Yet, he added, in contrast to the CIA case, many members of the FBI have been caught up in a grand jury probe that he likened to "the witch hunts of [the late Sen. Joseph R.] Joe McCarthy in the 1950s."

Leaks to the press from "unamed 'informed sources' within the Department of Justice" about the course of the investigation have "undermined the legal foundation upon which a grand jury probe is conducted," Decker charged.

These leaks, he said, have endangered rights of those involved to a fair trail, caused "anguish" for their families and seriously eroded morale of the FBI's 19,500 employees. He added:

"I cannot help but comment on the irony of the situation. FBI agents are accused of violating the civil rights of persons while attempting to enforce the law. Thereafter, the government, in seeking to investigate and enforce the law against those FBI agents, tramples upon it civil rights - the very persons whom they would accuse of violating the civil rights of others."