WASHINGTON — The son of an American held in North Korea sees a glimmer of hope after months of frustration and sadness that a planned summit of President Donald Trump and Kim Jong Un can help win his father’s freedom.

Sol Kim says he remains in the dark about why his father, a 59-year-old Korean-American, was detained at Pyongyang airport on April 22 last year after a monthlong stint teaching accountancy at a university in the North Korean capital. The father, Tony Kim, is one of three Americans North Korea is holding.

“I’m hoping the issue will come up and it will be discussed, maybe prior to the summit,” Sol, a 27-year-old grad student in California, told The Associated Press in a phone interview. “I’m just really hopeful. It’s a positive time.”

Trump and Kim are expected to meet in May, after more than a year of intense hostility over North Korea’s nuclear weapons development. Lingering in the background are the cases of Americans in custody, who have been denied consular access since their arrests over the past two years. Their detention has compounded the fraught relations between Washington and Pyongyang.

The North’s state news agency reported in May that Kim, whose Korean name is Kim Sang-duk, was arrested for committing “hostile criminal acts with an aim to subvert the country.” As far as Sol Kim knows, his father hasn’t been formally charged or put on trial.

For a family member of a detained American, Sol has some unusual insight into what life is like inside North Korea. A couple of years ago, he spent a month in Pyongyang as his father’s teaching assistant at the Pyongyang University of Science and Technology. The college was founded in 2010 on donations from Christian groups and is unique for its significant foreign staff.

“The students worked hard. They are eager. They are curious,” Kim said, recounting that he would spend time after class playing sports and sharing a meal with them. “It was a positive experience.”

Kim’s father was based in neighboring China, where he taught at a university and where the family lived for years. Tony Kim made at least seven trips to North Korea to teach. His wife accompanied him on the visit when he was arrested. She was allowed to leave the country.

Sol said his time in North Korea made it a little easier for him to comprehend his father’s situation, although he still doesn’t understand why his father is being detained.

Another detainee, agricultural specialist Kim Hak Song, taught at the same university. He was taken about a month after Tony Kim, similarly suspected of “hostile acts.” It’s not clear whether he has been sentenced.

The third and longest-serving prisoner, Kim Dong Chul, is a former Virginia resident who was reportedly the president of a trade and hotel services company in Rason, a special economic zone on the North Korean border with Russia. He was sentenced in April 2016 to a decade in prison with hard labor for espionage.

After the death last year of U.S. college student Otto Warmbier, Trump banned Americans from traveling to North Korea. He highlighted Warmbier’s case in his State of the Union address last January. Warmbier spent 15 months in North Korean custody for stealing a propaganda poster.

“Releasing the three prisoners would be a smart way for North Korea to improve the atmospherics and demonstrate goodwill” before the summit, said Evans Revere, a former senior State Department official for East Asia. The cases, he said, still “cast a pall over relations.”

The State Department declined to comment on the cases, citing “privacy considerations.”

As the U.S. has no diplomatic relations with North Korea, it relies on Sweden to help Americans inside the country. North Korea’s foreign minister visited Sweden last week, but it’s unclear whether Swedish diplomats will be able to visit the U.S. detainees.

Sol Kim, who speaks weekly with the State Department, said the only word the family got from his father was when a U.S. envoy visited Pyongyang in June to retrieve Warmbier just before he died. Tony Kim, they were told, appeared healthy.

Asked what he’d like to tell the North Korean government, Sol chooses his words carefully.

“I’m not really sure what to say,” he said. “We miss our dad. My mum misses her husband. We are worried for him and we are concerned for him.”