The company’s tours have some built-in protections. Its more than 30 group departures to North Korea follow the prearranged itinerary, are accompanied by at least two state-registered North Korean guides and a Koryo staff member and require travelers to abide by rules such as the prohibition against taking photographs of the military.

Lupine Travel says it is not exactly throwing caution to the wind, either. After Mr. Warmbier’s death, its managing director, Dylan Harris, said that United States citizens are no longer permitted on private tours because they would be accompanied only by North Korean tour guides and none of Lupine’s staff. “Therefore, we believe the risk is now too high without one of our staff alongside to advise throughout the trip and monitor how they are behaving,” he said. Americans can, however, go on its group tours.

Mr. Harris said that Lupine Travel had taken roughly 2,500 travelers to North Korea since the company began in 2008 without any incidents, but acknowledged that United States citizens had to be more on guard than people from other countries. “ “Any other nationality that commits a misdemeanor in North Korea is likely to either be ignored or result in a reprimand or deportation,” he said, “but a U.S. citizen is much more likely to face arrest and jail for the same thing.”

Mr. Warmbier was taken into custody in 2015 because he tried to remove a propaganda poster from a hotel wall, according to the North Korean authorities.

It is unclear if that is true, but one traveler said he understood the impulse. John, a retired investment banker who is a United States citizen living in Budapest and would speak only on the condition that he be identified solely by his given name, violated the rule of not taking pictures of the military when he went there in 2014. “I took a picture of soldiers marching by, and a government agent came up to me and grabbed me by the neck and screamed,” he said. “She let me go, but what happened with Otto could have easily happened to me.”

Still, John said he left North Korea “wowed” by the country.

Americans are strongly discouraged from traveling to North Korea. The State Department warns that United States citizens in the Democratic People's Republic of Korea “are at serious risk of arrest and long-term detention under North Korea’s system of law enforcement. This system imposes unduly harsh sentences for actions that would not be considered crimes in the United States and threatens U.S. citizen detainees with being treated in accordance with ‘wartime law of the D.P.R.K.’ ”

The warning adds that “being a member of a group tour or using a tour guide will not prevent North Korean authorities from detaining or arresting you.”