Congressman Bill Richardson told a news conference in Seoul he was hopeful that Chief Warrant Officer Bobby Hall would be freed in time for Christmas.
But he said he had received no direct assurances from Pyongyang on the release during five days of what he said were often "intensive, heated and hectic" negotiations.
Richardson said he understood Hall was being held at a military base near the site of Saturday morning's crash, a short distance north of the border.
His requests to see Hall or speak to him by telephone were denied, but the North Koreans agreed to pass on a note from his wife and gave assurances that he was "in good shape."
"What is holding up the release is the investigation the Korean People's Army is undertaking," Richardson said. He said he thought the military talks would establish the fact that the helicopter's presence over North Korea was accidental, and Hall would then be freed.
U.S. officials have said the helicopter probably strayed into North Korean territory after the pilots became disoriented in heavy snow. Pyongyang officials have accused the helicopter of launching a "surprise attack."
U.S. Defense Secretary William Perry told reporters earlier Thursday that intense negotiations were under way with North Korea over Hall's release. "I will say I have some hopes that it will happen soon," he said.
Earlier, Richardson, who acted as President Bill Clinton's envoy on the incident after arriving in Pyongyang on a previously scheduled trip last Saturday, brought the remains of U.S. Army Chief Warrant Officer David Hilemon to South Korea through the heavily fortified demilitarized zone that separates the two countries.
Richardson said the North Koreans had claimed Hilemon died as a result of injuries from the crash, and that his preliminary impression after examining the remains was consistent with that explanation.
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