US Army Signal CorpsDak To, South Vietnam. An infantry patrol moves up to assault the last Viet Cong position after an attempted overrun of the artillery position by the Viet Cong during Operation Hawthorne.During the US campaign in Vietnam, over a thousand service members were classified as prisoners or war (POW) or missing in action (MIA). Generally, those who went missing were killed or captured by the North Vietnamese.
But in a few instances, as Robert Beckhusen recounts at War is Boring, Americans were captured or went MIA when their planes were shot down in or near Chinese airspace. Beijing's role in the conflict is becoming increasingly apparent as the NSA releases signals intelligence (SIGINT) documents from Vietnam and Southeast Asia.
For instance, on September 2o, 1965, US Air Force Captain Philip E. Smith accidentally flew over the Chinese island of Hainan after his navigation equipment stopped working and he became lost in heavy cloud cover. A Chinese MiG-19 intercepted and shot down Philips due to his violation of Chinese territory.
According to NSA SIGINT reports, intercepted Chinese communications indicated that Beijing knew the identity of the plane that had been shot down and even recovered its pilot. One Chinese message stated that "On 20 September 1965, the Chinese Naval Air Force brought down a US imperialist F-104 fighter plane at one stroke over Haikou area of Hainan Island, capturing alive the pilot, a US captain." 
Screenshot/NSA
In another incident, Chinese MiGs were credited with the downing of a US aircraft, leading to the capture of Lt. Junior Grade Terrence M. Murphy and a second pilot whose name is redacted. The US aircraft downed one Chinese MiG before crashing into an unidentified body of water with both pilots' fates unknown.
At the time, Communist China and the US did not have diplomatic relations since the US considered nationalist Taiwan to be China's sole legitimate government. This may have made these confrontations more likely and made it harder for the US to negotiate for captured personnel by keeping the US and China on a mutually hostile footing.
The US also entered the war in Vietnam with the partial objective of countering the influence of China in a country that seemed ripe for a communist takeover. The direct confrontations the NSA documents were perhaps a logical consequence of the US's involvement in the conflict at all.
In an analysis of both incidents, the NSA released a report on Dec. 11 titled "Shootdown of US Aircraft Over Hainan Island." The report notes that "as many as ten CHICOM fighters [redacted] reacted to the hostile aircraft over Hainan Island. [redacted] this reaction to have been an aggressive one clearly intent upon downing the hostile intruder."
The report goes on to mention that the shoot down was a sharp departure from past policy, possibly signaling a more active Chinese policy of engagement against the US in the region.
As China began to play a more active role against US aircraft in the region, Beijing also captured American ground troops in Laos, according to the report "Communist Chinese Capture American Soldiers in Laos; Laotian Refugee Reports."
The report states that "[t]he Red Chinese went back to Shwe Hsaing ((B)), Laos and encamped at a site six miles from Shwe Hsaing. It is reported that (B percent Red) Chinese from that encampment (B percent captured) two American soldiers."
The NSA compiled the documents from over 7.5 million reports in the early 1990s in a bid to "assist in resolving the POW/MIA issue." The agency is now releasing a trove of 1,600 of these documents over the coming months and released the first 170 of them on Dec. 11.