A North Korean soldier stands guard along the Chinese border. (GOH CHAI HIN/AFP/Getty Images)

The State Department has long cautioned Americans about visiting North Korea, but on Tuesday it went a big step further, issuing a blanket warning against all American travel to the country. This was the first such State Department warning since North Korea began allowing American tourists in 1995, immediately raising the question: why?

The travel warning cited, somewhat cryptically, "the risk of arbitrary arrest and detention of U.S. citizens in North Korea." It noted that two Americans traveling on valid visas have been previously arrested. But neither of those was especially recent. Eddie Jun Yong-su was arrested in November 2010, allegedly for illegal missionary work, and released in March 2011. Kenneth Bae was arrested in November 2012 on similar charges and is still being held.

The travel warning does not explain what, if anything, has happened since last November that led the State Department to elevate its warning. A State Department spokesperson said that they could not comment due to U.S. privacy laws but emphasized that travel warnings of this severity are typically in response to "chronic" threats to U.S. citizens. Some early, unconfirmed reports are emerging that an elderly American man may have been detained.

A rising number of Americans visits North Korea every year on heavily orchestrated, state-monitored tours, a source of hard currency for the government there. The vast majority travel without incident. But, as NKNews.org editor Chad O'Carroll explained earlier today, the potentially lucrative business has attracted new tourist companies, some of which have little experience with North Korea's complex and highly sensitive restrictions. A source in the North Korean tourism industry suggested to O'Carroll, "Tourists traveling with some of the newest companies could be more likely to unwillingly fall afoul of North Korean laws."

A little before noon, U.S. Eastern Time, the San Jose Mercury News reported that an 85-year-old man from Palo Alto "has been detained in North Korea for more than three weeks" after North Korean authorities removed him from the plane that was to fly him out of the country. The report identifies the man as Merrill Newman, which is significant, as previously arrested Americans have been of Korean descent. The story also quotes a State Department spokesperson as declining to confirm or deny the story and saying only, "We are aware of reports that a U.S. citizen was detained in North Korea."

A newsletter produced by Newman's retirement home in Palo Alto indicated his intention to visit North Korea, NKNews' O'Carroll reports.

The reports saying that Newman has been detained in North Korea are, to reiterate, unconfirmed. But even if they were confirmed, two mysteries would remain: why did North Korea authorities take Newman into custody and how has it remained secret for a full three weeks? North Korea has strict rules for visiting foreigners, especially Americans, whose visits are closely monitored. Some Americans not of Korean descent have been taken into custody previously, but typically for highly sensitive political activities, such as religious evangelizing or, in the cases of journalists Euna Lee and Laura Ling, sneaking into the country without visas. It's not at all clear what Newman would have done.

For now, the only thing we know for certain is that the State Department has significantly escalated its warning against any American travel to North Korea, something usually done only in response to specific events, but that it has not explained why.