High officials in the Nixon administration testified Monday there was credible evidence at the time that American prisoners of war were being left behind in Indochina even as U.S. troops were pulling out of Vietnam almost two decades ago. Former secretaries of defense James Schlesinger and Melvin Laird also testified before a Senate committee that military and intelligence officials were angered by President Richard Nixon's claim in March of 1973 that all American prisoners of war and missing in action had been accounted for. The two former Nixon administration officials were the leadoff witnesses in the latest round of hearings by the Senate's Select Committee on POW-MIA Affairs. The committee is trying to determine if the Nixon administration tried hard enough to find out if Americans were still being held captive in Vietnam and Laos after U.S. troops were withdrawn from the region in 1973. Evidence and testimony already gathered by the committee has convinced some of its members that Nixon and his chief peace negotiator, Henry Kissinger, were not diligent enough in pressing for a full accounting of American prisoners or missing soldiers in Indochina. Schlesinger and Laird said the evidence indicating the Vietnamese and Laotians had captured American soldiers but had not listed them as POWS by 1973 consisted mainly of letters to relatives, unsubstantiated eyewitness sighting reports and radio contact with the men just prior to their capture. Some of the letters and sighting reports dated as far back as 1966. They said that much of the evidence was fascinating but that none of it proved conclusively the American captives were still alive after the mass release of U.S. POWs known as "Operation Homecoming" in the spring of 1973. Even so, the two men said many Nixon administration officials at the time believed at least some American servicemen, mainly pilots whose planes had been shot down, were still alive and being held captive. Nixon announced March 29, 1973, that "all of our American POWs are on their way home." Two weeks later, a Pentagon official, Roger Shields, said there was no evidence of Americans still being held captive either in Vietnam or Laos. Schlesinger testified that Nixon's statement triggered "a great deal of anger and frustration, particularly at the Pentagon but also at CIA (Central Intelligence Agency) among people involved in the process at the time. "A lot of these people had been in Vietnam and some of the POWs were their buddies," Schlesinger said. Laird, who had stepped down as secretary of defense shortly before Nixon's claim, said: "People over in the White House have a tendency to make positive statements. But I think it was unfortunate to be that positive." Asked point-blank if he believed Americans were left behind as captives as U.S. troops pulled out of Vietnam, Schlesinger answered: "As of now, I can come to no other conclusion. But that doesn't mean that any are alive today. The prospect of that would be very slim." Sen. John Kerry, D-Mass., chairman of the Senate committee, said the testimony by Schlesinger and Laird tended to support a growing impression that more could have been done to get back all American POWs. "I think it's quite extraordinary when two former secretaries of defense both give evidence documenting that they had information, or they believed personally, that people were alive and not accounted for in Operation Homecoming," Kerry said. The POW-MIA committee opened hearings this year to determine what happened to the 2,266 Americans listed as missing in action in Indochina. Almost all of those are now reliably believed to have been killed even though their remains have not been returned. The Senate hearings and an independent investigation being carried out by John W. Vessey, former chairman of the Joint Chiefs of Staff, are mainly concerned with the fate of 135 of the men believed to have been captured by the Vietnamese or Laotians who were not officially listed as POWs. Vessey said week before last his evidence indicates that about half of the 135, known as "discrepancy cases," died during or after capture. And there is no credible evidence, he said, that any of those who might have survived are still alive today. The Senate hearings are scheduled to continue today with testimony from Kissinger.