KUALA LUMPUR—Ri Jong Chol, a slight man in his mid-40s, led what appeared the routine life of a businessman. He lived in an apartment complex with a pool and gym, a family man who took his wife and two children bowling.

Mr. Ri’s computers and phones, seized last year by Malaysia authorities, reveal much more. He was a North Korean operative, one of hundreds living abroad who U.S. and United Nations investigators say help the regime skirt sanctions by generating cash and sourcing goods.