A Justice Department task force's investigation has found no evidence of FBI involvement in the 1968 murder of the Rev. Martin Luther King Jr., despite the bureau's earlier attempts to harass and discredit the civil rights leader.

Instead, the task force concluded, the FBI reacted to King's death by conducting a painstakingly thorough and honest search for his killer.

These are the main findings of the five-member task force's 149-page report, which was made public yesterday. The task force reached the conclusions after reviewing more than 200,000 documents in FBI files and interviewing approximately 40 witnesses.

The special probe was ordered last April by then Attorney General Edward H. Levi following press and congressional disclosures that the FBI, under orders from the late Director J. Edgar Hoover, had waged a five-year campaign of intimidation and surveilance against King.

Levi told the task force to determine whether there had been any cover-up or other improprieties in the FBI's probe of the King murder. He also directed the panel to look for any new evidence pointing to a conspiracy in the assassination.

The task force found that the evidence is surveyed virtually rules out the existence of a conspiracy. Instead, it said, the evidence points strongly to the conclusion that James Earl Ray, who pleaded guilty to the crime, acted alone in the shooting of King onma Memphis motel balcony on April 4, 1968. Ray is serving a 99-year sentence in a Tennessee state prison.

"The task force is satisfied that the FBI did a credible and thorough job in attempting to identify any possible conspiracy," the report said. "In all the years following the assassination, the investigation has failed to reveal any connection between any alleged cosnpirator and James Earl Ray."

"The sum of all of the evidence of Ray's guilt points to him so exclusively that it most effectively makes the point that no one else was involved," the report added.

Despite this unequivocal tone, the task force's report is not expected to lay to rest all the questions and theories about conspiracy surrounding King's death. That point was made by the new Attorney General, Griffin B. Bell, when he suggested in a recent television interview that the Justice Department probe neither established nor ruled out a consipracy.

The point Bell was trying to make, a department spokesman said yesterday, was that people are still going to read whatever they want to into th findings of the task force. In addition, the spokesman added, "the Attorney General was taking note of the fact that the report leaves was questions unanswered."

In Atlanta, the report was immediately denounced by Tyrone Brooks, a spokesman for the Southern Christian Leadership Conference, the civil rights group formerly headed by King. He said:

"We don't have any faith in that report because it seems the FBI and the Justice Department, to a degree, were involved in the assassination or the events surrounding it."

Also in Atlanta, King's widow, Coretta King, said she wanted an independent agency to investigate "whether the Justice Department made a sufficient inquiry and could find their conclusions convincing."

J. Stanley Pottinger, the outgoing assistant attorney general in charge of the Civil Rights Division, also had suggested to Levi that an independent panel of persons outside government should review the task force's findings. Justice Department sources said yesterday they did not know whether Bell has any plans to follow that recommendation.

The House has established a committee to investigate the killings of King and President Kennedy, although it currently is bogged down in a bitter fight between its chairman, Rep. Henry B. Gonzalez (D-Tex,), and its cheif counsel, Richard A. Sprague.

Some of the allegations that prompted establishment of the House committee are addressed in the task force report. These involve questions by black civil rights leaders about the withdrawal of a black detective, who had been assigned by the Memphis police to keep King under surveillance, and the transfer of two black fireman from a firehouse across the street from the motel where King was shot.

The report confirms that the detective, Edward E. Redditt, who had taken up a post in the firehouse on the day of the murder, was ordered to go home at 4 p.m. because of a reported attempt on the life of his family. The order was attributed to Memphis Police and Fire Department Frank C. Holloman, who had served in the FBI for 25 years.

The task force said Redditt may have been removed becuase Philip R. Manuel, an investigator for the Senate Government Operations Committee, apparently notified Memphis police that his staff had received a tip from an informer about a plot to kill a "Negro lieutenant" in Memphis.

Manuel, the task force said, has no "present recollection" of the incident. However, the report added, "He said the events sounded familiar and he believed the Memphis police records were correct."

The report also noted that another black police officer, Willie B. Richmond who also had been assigned to surveillance of King, remained in the firehouse after Redditt's departure and was among those who rushed to the motel after the shot was fired.

The task force said it could not establish clearly the reasons why the two black firemen, Norvell E. Wallace and Floyd E. Newsum, were transferred from the firehouse. It did note, though, that both were strong supporters of the strike by Memphis sanitation workers, most of whom were black, that had drawn King there as a gesture of solidarity.

"Our investigation has not disclosed any evidence that the detail of Wallace and Newsum was in any way connected with the assassination," the report said. It speculated that they might have been transferred because their sympathy for the strikers would have made hostile to what many blacks saw as police efforts to syp on King.

Perhaps the biggest question left unanswered by the report is where Ray got the substantial sums of money that he spent during the year between his escape from a Missouri prison and King's killing. The task force said, "The sources for Ray's funds still remain a mystery today."

In regard to the Hoover-inspired FBI harassment of King, the task force found that there might have been legitimate reasons for investigating him initially, because the FBI had established that one of his advisers at one time had had Communist ties.

However, the report continued, it quickly became clear that King was not under any Communist influence, and the security investigation should have been ended. To continue it for five years with frequently illegal tactics was "clearly improper," the report concluded. The King investigation started under Attorney General Robert F. Kennedy and continued under his successors, Nicholas Katzenbach and Ramsey Clark.

It noted that criminal charges against FBI participants in the harassment were barred by the five-year statute of limitations.