A U.S. toxicologist researching incidents involving the doomsday cult Aum Shinrikyo met one of its former senior members on death row on Thursday to discuss the recent murder of Kim Jong Nam, the North Korean leader's half-brother, by VX, a deadly nerve agent.

Anthony Tu, professor emeritus at Colorado State University, said the murder of Kim Jong Un's estranged half-brother was one of the topics discussed during his 30-minute meeting with Tomomasa Nakagawa at the Tokyo Detention House.

Nakagawa, 54, wrote a letter to Tu on Feb. 22 saying Kim Jong Nam may have been attacked with VX. He wrote the letter two days before Malaysian police announced that the nerve agent was detected in his body.