Dallas police officials have challenged the evidence that a second gunman fired at President Kennedy in Dallas 15 years ago, but House investigators yesterday held firm to the conclusion.

They said the Dallas police contentions fail to "lay a glove" on the scientific studies underlying the finding.

The dispute involves a Dallas police recording of the sounds coming from a motorcycle patrolman's stuck transmitter when Kennedy was killed in Dallas' Dealey Plaza on Nov. 22, 1963.

Backing off from congressional testimony he gave last week, Dallas police officer H.B. McLain, whom the House Assassinations Committee identified as the one driving the motorcycle, insisted in Dallas Thursday that the recording couldn't have been his.

"If it had been my mike, you would have heard sirens," he told reporters. He said as soon as Kennedy was hit, officers in the motorcade were radioed to escort the Kennedy limousine to Parkland Hospital. McLain was quoted as saying he stayed close to the motorcade for the entire highspeed trip.

"All the sirens were on," McLain asserted. "I couldn't distinguish one from another, but I know mine was going."

The chief counsel for the House Assassinations Committee, G. Robert Blakey, said yesterday, however, that McLain's memory on that and other points was plainly wrong. Photos taken moments after the assassination by a United Press International photographer, Blakey said, show that McLain and another motorcycle officer were still in Dealey Plaza alongside the grassy knoll after the motorcade had sped off ahead of them.

McLain "told us that was him" Blakey recounted. Blakey and deputy chief counsel Gary Cornwell said McLain must have forgotten to put his siren on, then caught up with the motorcade where siren sounds can be heard on the recording, about 1 minute and 50 seconds after the shooting.

"He says he only heard one shot, too," Blakey added of McLain's congressional testimony. "His memory is just no good." Blakey pointed out that McLain also told the committee last week that his microphone might have become stuck open because this happened on his old Harley-Davidson so often "that I'm scared to say."

Two acoustics experts, Mark Weiss and Ernest Aschkenasy, told the House committee at the same hearing last week that their tests showed "beyond a reasonable doubt" that a second gunman fired at Kennedy's motorcade from the area of the so-called grassy knoll, in front of and to the right of the president.

A separate study done for the committee earlier showed that three other shots had been fired at the president from above and behind him, all from the direction of the Texas School Book Depository were Lee Harvey Oswald was said to have set up a sniper's nest.

"We never said whose mike it was," Aschkenasy emphasized yesterday. But he remained sure it was on a police motorcycle in Dealey Plaza, apparently turning from Houston onto Elm Street, about 120 feet behind Kennedy's car when the third of four shots rang out.

"We predicted when the echoes (from a grassy knoll shot) should have arrived at the mike and we found the Dallas police tape matched (that prediction) within one-onethousandth of a second," Aschkenasy said.

Disputing that finding, Capt. J. C. Bowles, head of the Dallas Police Inspections Division, said he was satisfied from his own study of the recording that the sound had been picked up by a motorcyle in the Dallas Trade Mart area some 2.2 miles from Dealey Plaza.

The Dallas police captain said, however that he used a copy of the recording made some time ago by the Columbia Broadcasting System, one that CBS newsmen have said differs markedly from the copy used by the House committee's experts. Blakey said the House committee at first also got a copy on which "no gunshots," in fact "no audio sounds" at all could be discerned during the few seconds when the shots rang out.

Finally the original Dictabelt was located and the experts made a copy from that. After listening to it and studying it exhaustively, Aschkenasy has said that if someone were to tell him the motorcycle transmitter were someplace other than Dealey Plaza, "I would go there -- and I would expect to find a replica of Dealey Plaza in that location."

The scientist who did the initial committee study, James Barger of the Cambridge, Mass., firm of Bolt, Beranek and Newman, agreed. If the motorcycle had been at the Trade Mart, he said yesterday, "the Trade Mart would have to resemble Dealey Plaza in very fine detail." In addition, he indicated, there would have to have been shots fired there, too.

Capt. Bowles said he felt the motorcycle must have been in the Trade Mart because of a variety of factors, such as the long delay before any sirens can be heard, but primarily because all cycles not assigned to the motorcade were supposed to go there. He also said that just "a quick thump" can produce a supersonic sound wave such as the one the experts detected and ascribed to a bullet. But Bowles agreed there was no evidence of any shooting at the Trade Mart that day.

"The reconstruction of the sounds is a matter of science," Blakey said. "If they want to refute what we did, they've got to redo the science and show us it's the science that's wrong... But if they say the oral testimony that McLain gave us is inaccurate, they can't lay a glove on us... That's like coming up with a witness who says he was playing cards with the defendant at the time of the killing when the defendant's fingerprint is on the knife sticking in the victim's back."