After a $5.4 million inquiry that was touted for its independence from the executive branch, the House Assassinations Committee wound up its work yesterday by asking the Justice Department to pick up the loose ends.

The 2 1/2-year congressional investigation into the murders of President Kennedy and the Rev. Martin Luther King Jr. concluded that both killings were probably the result of conspiracies, but the committee was unable to determine the extent of the plots or who, besides the original suspects, was involved.

"The findings of the committee in its two separate inquiries were remarkably similar," Chairman Louis Stokes (D-Ohio) observed at a news conference yesterday. "We were satisfied, in each case, with the determination of federal investigations in 1964 and 1968 that Lee Harvey Oswald and James Earl Ray were the respective assassins. We did not concur, however, with the conclusions that Oswald and Kay had acted alone."

The committee's final report, made public Tuesday, suggested tentatively that Kennedy's 1963 assassination may have stemmed from plotting by a bizarre assortment of gangland figures and anti-Castro activists. It also offered, in a footnote, the alternative suggestion that Oswald may have been aided by one or two other "left-wing" types who could have been his confederates in an earlier attempt on the life of Gen. Edwin A. Walker.

The committee said it considered Mafia leaders Carlos Marcello of New Orleans and Santo Trafficante of Tampa "the most likely" gangland leaders to have taken part in an anti-JFK plot, but then said it was "unlikely" that either had done so.

In the King case, the committee forcused on the possible involvement of Ray's brothers, John and Jerry, and on what it called "substantial evidence" of a St. Louis-based conspiracy featuring two segregationist businessmen who had reportedly made a standing offer of $50,000 for King's death. But the House panel acknowledged it could not be definite. It also sharply criticized the FBI for "investigative excesses," including the interception of James Earl Ray's jailhouse mail, that might have jeopardized the original investigation.

In any case, the committee wrapped up its work by recommending that the Justice Department "determine whether further investigation is warranted."

Stokes said he recognized that some would say "the committee should have pursued the plots to the point of assessing individual responsibility for them," but he defended the decision not to do so, partly on the ground that this was not the proper province of a congressional committee.

"We were determined to respect the rights of subjects of our investigation - including possible suspects in the conspiracies - and their associates," Stokes declared.

Asked how the committee had protected the rights of Marcello, Trafficante or Ray's brothers, Stokes insisted that "the committee has not, in any way, defamed or degraded anyone" with unwarranted conclusions.

"We tried to set forth the evidence as we found it," he said. "We did not have the right to come to any conclusions that were unwarranted." But he added that "we had to report to the American people."

A committee minority dissented from the findings, primarily over the acoustical evidence underpinning the conclusion that a second gunman fired at Kennedy from the so-called "grassy knoll" overlooking Dealey Plaza in Dallas. CAPTION: Picture, REP. LOUIS STOKES...findings "were remarkably similar"