An 85-year-old Palo Alto, Calif. man who served during the Korean War has been detained in North Korea, the man’s son said Wednesday. It’s not clear why he was detained and North Korea has yet to officially acknowledge it. Still, the U.S. envoy to North Korea is urging the government there to release him.

Jeff Newman said Wednesday that he and his family have not spoken with his father, Merrill Newman, since Oct. 26, reports CNN. He called the situation a “misunderstanding,” noting that his father had visited North Korea through an approved travel agency.

“He arranged this with a travel agent that was recommended and said was approved by the North Korean government for travel of foreigners,” Jeff Newman said. “He had all the proper visas.”

According to the San Jose Mercury News, just before he was scheduled to leave the country, Merrill Newman met with North Korea officials and they spoke about his war service. But the next day, when the plane was about to take off, he was pulled off.

Bob Hamrdla traveled with Merrill Newman and said Wednesday that it was a “a terrible misunderstanding.” He added, “I hope that the North Koreans see this as a humanitarian matter and allow him to return to his family as soon as possible.”

Experts think that it is strange that North Korea hasn’t acknowledged that Merrill Newman has been detained. But Glyn Davies, a special U.S. representative to North Korea, told reporters in Beijing Thursday that Pyongyang should release Merrill Newman and the Christian missionary Kenneth Bae, who is also detained.

“We of course are calling on North Korea, as in the case of Mr. Kenneth Bae, who has now been there for over a year, to resolve the issue, and to allow our citizens to go free,” Davies said, reports Voice of America.

“We are working very hard ... to try to move this issue along,” Davies further explained after meeting his North Korea counterpart. “We certainly think that North Korea should think long and hard about (this) and understand that for the United States, this is a matter of core concern for us.”