The last two American hostages aboard the hijacked Kuwaiti airliner arrived safely here today en route to West Germany and said they had seen no evidence of Iranian collaboration with the four terrorists involved in the six-day ordeal.

Speaking to reporters at Kuwait International Airport on their arrival with 18 other freed hostages, American businessman John Costa, 50, said he had "no evidence whatsoever" that Iranian authorities had helped the hijackers.

The other American, Charles Kapar, 57, an auditor for the U.S. Agency for International Development, said that "to our knowledge, we know of no cooperation with the Iranians."

The comments of the two Americans conflicted with the testimony today of two Pakistanis and added to the confusion over the Iranian role during the hijacking operation, in which two American officials were shot to death and one Kuwaiti injured.

(n Tehran, Ayatollah Ruhollah Khomeini, in his first public speech in nearly five weeks, denied that the Iranian government was involved in the hijacking, Reuter reported. He pointed out that the hijacking "was condemned by all Iranian officials and . . . solved in a satisfactory way."

The assertion of several other returning hostages here, including the British pilot, tended to substantiate the claims of the two Americans that there was little or no hard evidence of active Iranian cooperation with the four hijackers.

They either denied or were unable to confirm widespread allegations here and elsewhere that, once the aircraft arrived at Tehran airport, the Iranians had provided the terrorists with arms, handcuffs, ropes and explosives. Some Arab press reports even suggested that Iran had provided substitute hijackers to replace those on the plane when they became exhausted.

Kuwaiti officials privately, and the media here openly, have been charging that Iran was involved from the start, possibly even in the planning of the hijacking.

But the top Kuwaiti negotiator sent to Tehran to talk to the hijackers, Ahmed Ayoub, contradicted this tonight by telling reporters, "I don't think so" when asked if he thought Iran had aided the hijackers.

"I cannot say so," he added, describing Iranian cooperation with his negotiating team as "fine."

The British pilot of the Kuwaiti Airways Airbus, John Henry Clark, also refused to confirm charges that Iran had cooperated with the terrorists and denied reports that more guns had been brought on board after the airliner arrived in Tehran. He said he had seen two "relatively small" automatic pistols, two or three revolvers and "a couple of hand grenades" from the beginning and did not think others were put aboard after the plane landed.

Clark also said he suspected most of the explosives and wires planted in preparation for the threatened blowing up of the aircraft during the last night of the drama were "some sort of fake, one more attempt to frighten us."

Clark and the two Americans also said there were only four hijackers involved from the start and that they were the same persons throughout the ordeal.

Earlier today, two Pakistani passengers interviewed upon their arrival in Karachi gave a conflicting account, charging the Iranians had aided the hijackers materially.

"They had nothing when they board the aircraft at Dubai," Sheik Abdul Hafiz, a catering officer for Kuwait Airways, was quoted as saying by United Press International in Karachi.

"They had everything two days after arrival at Meherbad Airport in Iran," he added, mentioning specifically .38-caliber pistols, iron handcuffs and nylon rope.

Costa said he thought the handcuffs had been taken from a wounded Kuwaiti security guard aboard the plane, who also lost his pistol. But neither he nor Kapar could explain the presence of rope aboard the aircraft used to tie up some of the passengers. Part of the debate over alleged Iranian complicity has centered on the long delay before Tehran authorities ordered the storming of the plane. But some western observers here pointed out that the delay did permit them to extricate all but seven passengers -- four Kuwaitis, two Americans, and the British pilot -- before they took any action.

None of the passengers, and apparently none of the hijackers, was injured by bullets during the Iranian takeover of the plane.

The two freed Americans, obviously delighted to be alive and out of Iran, appeared in good spirits as they were mobbed by reporters and Kuwaiti officials as they welcomed the specially chartered Kuwait Airways plane.

Both had black eyes and faces covered with cigarette burns from being beaten and tortured by the hijackers, who they said were trying to extract confessions that they were working for the CIA.

Kapar, whose family lives in Arlington, Va., said it was hard to describe how he felt about the experience except to say, "We were terrorized from morning to night. . . . It was 140 hours of constant terror, 140 hours of hell."

"I thought I would die every day," he said. But, he added, "I never lost hope." Karpar said the six days aboard the aircraft were "like a prison," with "ups and downs" in the behavior of the four hijackers, "depending on how events of the day went."

Costa said he was asleep in row 13 of the economy-class section a few minutes after the takeoff from Dubai when he heard shots.

Then, he recalled, "they started calling for Americans. So I stood up. It was obvious they were going to find out sooner or later. So the ringleader pulled me around a little bit, made me lie down on the floor, stood on my back and made a small speech about American imperialism."

Costa, a New York City resident, said the hijackers had looked for diplomatic passports to see if there was anyone of importance they could use to pressure the Kuwaiti government into releasing the 17 terrorists being held here for a spate of bombings last December.

The Americans and Kuwaitis were put together in one section, and "singled out for special attention," Costa said.

He said he heard two shots shortly after landing in Tehran Tuesday morning from inside the plane when the first American was killed. His body was dumped out of the plane onto the airport tarmac.

It was not clear from his account which of the two Americans -- Charles F. Hegna, of Sterling, Va., and William L. Stanford, a resident of Karachi, Pakistan, both auditors with AID -- was shot first.

The second was killed several days later on the steps of the aircraft after being made to count down from eight to zero and reportedly attempting to seize the hijacker's gun.