It was Sept. 20, 1965 when the navigation equipment aboard Capt. Philip Smith’s F-104 Starfighter failed.

Smith’s mission was to escort an airborne early-warning plane patrolling above the Gulf of Tonkin. Instead, his supersonic jet strayed over Hainan Island—through airspace belonging to the People’s Republic of China.

Chinese radars detected the incoming F-104. MiGs raced towards the American jet, shooting it down. Smith survived the incident. China imprisoned the captured pilot, freeing him in 1973.

Now there’s new information about the shootdown from the National Security Agency and the Pentagon’s Central Security Service. On Dec. 11, the agencies released 170 out of 1,600 soon-to-be-declassified documents involving Americans captured or deemed missing in action during the Vietnam War.

Many of the partially-to-heavily redacted documents are signals intelligence reports written in the aftermath of aircraft losses.

Most of the trove references American pilots lost over North Vietnam. But Chinese fighter jets intercepted and shot down American aircraft on several occasions, killing several pilots. It’s a little known and politically sensitive aspect of the war in Vietnam.

Some of the details are still classified.

“It appears that possible as many as 10 Chicom fighters … reacted to the hostile aircraft over Hainan Island,” noted a Sept. 20 report following Smith’s capture, using an abbreviation for Chinese communists.

“This shootdown of a U.S. aircraft intruding over Hainan Island represents a sharp departure of policy on the part of the Chicoms from that demonstrated during a similar intrusion of 9 April 1965,” the report added.

This is reference to an April clash between Navy F-4 Phantoms and Chinese MiG-17s. During the dogfight, a MiG gunned down an F-4, killing both pilots. The report suggests the Chinese were more aggressive during the later September incident, reflecting a “sharp departure of policy.”

“On this [April 9] occasion the Chicoms appeared to be exerting considerable effort to avoid an engagement,” the report states. “Although presented with apparently favorable circumstances for an attack on the intruding hostile aircraft.”

Complicating efforts to find out what happened during the September shootdown, a U-2 spy plane handling signals intelligence—known as Trojan Horse—was busy over Laos.