A former U.S. soldier said today that he saw a man identified as Dr. Josef Mengele at an American prisoner-of-war camp in Germany in 1945, the first witness to suggest that the notorious Nazi war criminal was once in U.S. hands.

Two U.S. senators at a news conference to hear the account by retired aerospace engineer Walter Kempthorne said they will insist that the U.S. government determine whether Mengele was in American custody and, if so, how he could have escaped it.

Mengele, who would be 73 if alive, was a physician and former major in the Nazi secret police who allegedly sent thousands of concentration-camp prisoners to their deaths in gas chambers and used others, including many children, in painful medical experiments. Considered the most notorious Nazi war criminal still at large, Mengele is wanted in West Germany on murder charges and is thought to have been hiding in South America since the war.

Sen. Arlen Specter (R-Pa.) said at the news conference at the Simon Wiesenthal Center that investigations of U.S. dealings with other former Nazis raise the possibility that U.S. officials may have helped Mengele escape in exchange for information about Soviet activities in Europe.

"I think Mengele is alive. I think the noose is tightening," said Sen. Alfonse M. D'Amato (R-N.Y.), who praised a recent Justice Department decision to begin a special investigation of the Mengele case. But, he added, citing a Freedom of Information Act suit that he has filed against the U.S. Army, "the release of all pertinent documents should be the order of the day."

Army spokesman Lt. Col. Craig MacNab said Kempthorne's report "is brand-new information which we welcome." The Army recently has released other documents, one of which suggests Mengele once lived in Canada, and MacNab said it will work with the center and other officials to pursue the lead. "The problem is an embarrassment, not a lack, of records . . . . They all have to be gone through by hand," he said.

Kempthorne, 59, of Riverside, Calif., said he wrote to Rabbi Marvin Hier, dean of the Wiesenthal Center, after reading about the center's earlier disclosure that a U.S. intelligence officer thought Mengele had been briefly in U.S. custody in 1947.

Kempthorne said he was serving as a perimeter guard at an Army Counter-Intelligence Corps post at Idar-Oberstein, 50 miles east of Trier in southwestern Germany, when he encountered the man identified as Mengele.

A friend who had a habit of trading favors with other soldiers invited Kempthorne to help deliver some liquor or cigarettes to a guard inside the post. There he said he saw what appeared to be a German prisoner standing "at rigid attention, . . . he had a fixed look on his face . . . . He was breathing heavily and was red-faced."

In a letter to Hier released today, Kempthorne quoted his conversation with the man's two U.S. guards:

Kempthorne: "Geez, what are you guys trying to do to him? He's ready to fall over."

One of the guards: "We're getting him in shape to get hung. This here is Mengele. The bastard that sterilized 3,000 women at Auschwitz. (Turning to the prisoner) C'mon, boy, you're good for another 100 . . . . "

On his guard's command, the prisoner dropped to the ground to do more pushups, but was too exhausted and was led away, Kempthorne said.

Kempthorne said he does not remember the prisoner's face, but noted he had black hair and was about 5 feet 8 and 165 pounds, which Hier said nearly matches Mengele's description. He remembered the prisoner wore "shell- or horned-rimmed glasses."

Hier said pictures taken of Mengele before the war show him without glasses and Kempthorne's failure to remember facial characteristics might make identification from old photographs difficult. Hier said he hoped publicity about the case and the search of Army records would uncover more witnesses or data on the fugitive Nazi.

Kempthorne said he has occasionally thought of the incident when reading about Mengele since then, but did not know who might be interested in hearing his story until he read of the center's investigation.

"I always visualize the name of Mengele with the face of the man I saw in that camp because I connect it with 3,000 sterilized women," Kempthorne said. "That's a mental image that is hard to forget, especially for a 19-year-old kid."