The House Assassinations Committee has received startling last-minute evidence that there was a conspiracy in the assassination of President Kennedy 15 years ago.

The Committee was told at a closed meeting Monday night that refined acoustical tests showed-"beyond a resonable doubt" in the words of one source-that a gunman fired at the president in Dallas on Nov. 22, 1963, from the area of the grassy knoll in Dealey Plaza.

"The Warren Commission blew it," a committee source said of the central finding in 1964 that Lee Harvey Oswald, acting alone, killed the president. "The commission said there was no scientific evidence of more than one gunman . . . and therefore there was no conspiracy.

"We've established that there was a conspiracy. If we can't identify the second gunman, that's because it's 15 years later. I don't know what you do about that now."

The House panel is in process of wrapping up a two-year, $5 million investigation of Kennedy's assassination.

The Warren Commission said in 1964 that three shots were fired at Kennedy, all from behind, all from the sixth floor of the Texas School Book Depository, where it said Oswald had set up a sniper's nest.

Scientific evidence of a fourth shot, fired at Kennedy from the grassy knoll in front of the presidential limousine, was submitted to the committee this week.

Rep. Harold Sawyer (r-Mich.) made that disclosure yesterday during a radio interview in Grand Rapids, Mich., about the committee's investigation. The House inquiry is scheduled to end Dec. 31.

Sawyer said the committee was told by two acoustical experts "that there were four shots, the third of which was fired from the grassy knoll."

The experts said there was a 95 percent probability to the finding.

The evidence was said to be contained in a long-ignored tape of the sounds in Dealey Plaza. The recording was made on a Dallas police radio frequency when a motorcycle patrolman left his transmitter on.

"They [the experts] can tell within two feet of where the [fourth] shot came from," a committee source said. "They said, beyond a reasonable doubt, that there was a gunman on the grassy knoll . . . they say it [the noise from that area] is a shot, probably a rifle shot, because it has a supersonic wave, a shock wave preceding the speed of sound" such as rifles make.

Sawyer said on the Grand Rapids radio show that he couldn't remember the names of the experts who reached the conclusion, but other sources identified them as Mark Weiss and a colleague from Queens College in New York, both trained in acoustical and electrical engineering. Weiss was a member of the court-appointed panel that analyzed the famous gap on one of President Nixon's Watergate recordings.

An initial study of the Dallas police radio recording was made for the House committee earlier this year by James Barger, an acoustical expert with the Massachusetts firm of Bolt, Bernack and Newman. In testimony before the committee last Sept. 11, he said his work on the tape showed a 50-50 possibility of a fourth shot, from the grassy knoll area.

Weiss and his colleagues were commissioned by the committee to see if they could reach a more definite conclusion, one way or the other. They concentrated on the third of the four noises that Barger's studies had identified as possible gunshots.

The third had come from the grassy knoll area where photographic evidence-by itself inconclusive-shows an individual behind a fence with a linear object next to him, an object that could not be identified further.

Concentrating on the noise from the grassy knoll, Weiss and his colleague plotted the echo pattern mathematically and traced every building and other object off which it bounced in 1963.

The work was so precise, one source said, that "they could tell us there was probably a boxcar on the [nearby] railroad tracks. They even identified the press bus coming around the corner."

They also reportedly fixed the position of the motorcycle and even determined the side of the bike where the microphone was located.

As part of the study; the experts obtained an old, presumably similar, microphone and motorcycle and conducted test firings in New York with the help of New York police. From those tests, sources said, "they were able to reproduce the sound on the [Dallas] tape."

Sawyer said that Barger studied the new data and agreed with the findings. "Barger was there [at the Monday night meeting] and he was totally in concurrence," Sawyer said.

The Michigan Republican said he was "probably in hot water" for disclosing testimony taken in executive session, but he said he found himself under close questioning during the radio show.

"There was no way I could answer them," he said of his interviews, "without getting into this."

The new findings also leave the committee in a quandary. With its final report due within the nect two weeks, Sawyer said, "I don't know how in the name of heaven we are going to handle this . . ."

Only three cartridge cases were found near the window near the sixth-floor window of the book depository where Oswald had been placed. The Warren Commission said that one bullet missed, another hit President Kennedy in the back of the neck and then wounded Texas Gov. John Connally and the third struck the president in the head.

"There is no credible evidence that the shots were fired from the triple Underpass, the head of the motorcade, or from any other location," the Commission concluded. "The weight of the evidence indicates that there were three shots fired."

There is still likely to be considerable debate over whether the shot from the grassy knoll hit the president or not. The House committee may take the position that it missed the presidential party.