North Korea may be returning around 50 sets of remains from USA soldiers who went missing during the Korean War. The average annual income of North Koreans was just 5% of that earned by people in the South, the central bank estimated.
Prosecutors said they are expanding the probe to determine if there were any others involved.
North Koreans cry while waving to their South Korean relatives as they head back to South Korea after reuniting at the Diamond Mountain resort in North Korea.
"As long as exports of minerals are part of the sanctions, by far the most profitable item of its exports, Pyongyang will have no choice but to continue with its current negotiations with the US", said Kim Byeong-yeon, an economics professor at the Seoul National University who specializes in the North Korean economy.
North Korea's Gross Domestic Product (GDP) dropped 3.5 percent in 2017, the biggest drop since the 6.5 percent drop in 1997.
Trump - no hurry on North Korea denuclearisation
Repatriating remains of United States soldiers who perished during the 1950-53 conflict was part of a written agreement signed by Kim and Trump during their landmark summit in Singapore last month.
According to US military newspaper Stars and Stripes, North Korea has agreed to return as many as 55 sets of remains believed to be USA soldiers killed in the Korean War.
Trump on Tuesday said that there is no hurry to denuclearize North Korea under his accord with Kim - a shift in tone from when the U.S. leader said the process would start very soon.
National Security Adviser John Bolton said more recently that Washington has a plan that would lead to the dismantling of the North's nuclear and missile programs in a year. The leaders had agreed to work towards "complete denuclearisation" of the Korean peninsula.
This comes at a time after North and South Korea agreed to hold a family reunion event by mid-August this year, as decided the two leaders during the historic inter-Korean summit at the Demilitarised Zone (DMZ) on April 27 this year.
North Korean leader Kim Jong Un made a broad commitment to denuclearize at an unprecedented summit with Trump in June, but offered no details as to how and when this might take place, leaving considerable doubts about Pyongyang's intentions.