In May 1968, naval intelligence informed the Joint Chiefs of Staff that the seizure of the USS Pueblo had caused “the probable compromise of a considerable amount of U.S. classified material.” The CIA reported in August that North Korean divers had “recovered the gear tossed overboard by the Pueblo crew prior to capture.” At a closed-door congressional hearing in September, CIA chief Richard Helms stated that the North Koreans “have been dismantling the antennas and by now have probably completed their exploitation of the ship’s equipment.”

Ever since the crewmen were debriefed in San Diego, NSA and Navy intelligence specialists had been working to assemble a detailed picture of how the Pueblo’s capture had affected national security. At Naval Security Group headquarters in Washington, about 75 analysts sifted through the sailors’ extensive statements in an effort to determine exactly what codes, code machines, and other materials had been compromised.

By late February 1969, the first of several top-secret damage assessments was completed. The cumulative conclusion was stark: the actual intelligence loss was far graver than originally believed. (These reports remained classified for many years. At the author’s request, the National Security Agency eventually declassified three of them—a process that took more than seven years.)

Of the 539 classified documents and pieces of equipment aboard the ship, up to 80 percent had been compromised, the NSA reported in one assessment. Only 5 percent of the electronic gear had been “destroyed beyond repair or usefulness.” As an NSA historian later wrote of the Pueblo seizure: “It was everyone’s worst nightmare, surpassing in damage anything that had ever happened to the cryptologic community.”

NSA experts described the crew’s attempts at destruction as “highly disorganized” and carried out “in almost total confusion.” LT Steve Harris was faulted for not drilling his men beforehand. The lieutenant told his debriefers he was “pretty confused” and “a little bit scared” as his men struggled to tear apart their gear. But instead of directly supervising and helping them, Harris spent his time anxiously overseeing radio transmissions to Kamiseya.

“I didn’t pay any attention to the emergency destruction,” he admitted in one damage estimate. “I would like to have, but I felt that there should be no unauthorized information transmitted because this was being watched very closely by high-ranking people.”

CDR Bucher, too, came in for criticism. Although the captain had tried in Bremerton and Japan to obtain a rapid-destruction system, an NSA report faulted him for not following through and “ensuring that his men knew what to do in an emergency.”

NSA investigators conducted an experiment to test the effectiveness of the crew’s frantic attempts to demolish code machines. Hefting sledgehammers and fire axes, they bashed similar encryptors with the same force and frequency described by the CTs. The metal-walled devices proved remarkably strong. The tightly wired circuit boards, the NSA team found, were largely immune to pounding. Even after being smashed apart, some components could be reassembled in as little as thirty minutes.

Moreover, the CTs had neglected to destroy spare parts. The NSA concluded it was “highly probable” that communist electronics experts, using spares and scarcely damaged circuit boards, were able to assemble working copies of at least three of the four types of code machines aboard the ship. North Korea’s acquisition of the devices was “a major intelligence coup without parallel in modern history,” according to an NSA study written in 1995.

The CTs had tried to destroy as many operating and maintenance manuals for the code machines as they could. To an enemy specialist, such handbooks could provide crucial insights into how the encryptors functioned, if not partial blueprints. But wastebasket fires consumed few of the manuals, and the two slow shredders weren’t even used. Sailors tossed some books into ditch bags and mattress covers, intending to jettison them when the fleeing ship reached the 100-fathom curve. Since it never did, an “undetermined number” of manuals were captured intact, the NSA reported. (One bag was heaved overboard, but the North Koreans retrieved it.) Publications stored outside the SOD hut had been “largely overlooked in the frantic destruction efforts.”

The damage evaluators struggled to determine precisely what had fallen into communist hands because the sailors had failed to keep track of what they burned or tore apart. For example, no one was sure what had become of the “key cards” — IBM-style punch cards used to program code machines each day. NSA investigators established that of 36 key card booklets aboard, only two had been fully destroyed. The rest, they were forced to assume, had been compromised.

It was clear that the loss included all code cards for November and December 1967 — materials that should have been burned within weeks of use but hadn’t been. These were particularly valuable, because an enemy that had intercepted and recorded encrypted messages during those months could use the captured cards to decrypt them. While the North Koreans likely lacked the technical capacity to exploit two months of worldwide U.S. radio traffic, the Soviets did.

Indeed, Moscow appeared to have benefited substantially from the spy-ship debacle. According to NSA sources, the North Koreans arranged for Soviet technicians to examine materials from the Pueblo “immediately after the seizure.” A group of electronic-surveillance specialists from Soviet military intelligence inspected the vessel, and several electronic components were taken back to the USSR for closer study.

American intelligence officials were also alarmed by the possible compromise of roughly 8,000 messages radioed to Bucher’s ship as part of the Navy’s fleetwide operational intelligence broadcasts in the Western Pacific. Transmitted from Guam, these so-called GOPI broadcasts carried detailed reports of U.S. air and ground combat in Vietnam, along with results of American intelligence-gathering efforts throughout Southeast Asia.

Source “Act of War”, by Jack Cheevers