Before he was executed in 2018 for his involvement in Aum Shinrikyo's 1995 sarin nerve gas attack on the Tokyo subway system, a former senior member of the cult wrote a letter in English urging would-be terrorists not to carry out their planned attacks.

"I want 'future terrorists' to stop their attacks on their own will before they are called 'real ones,'" Tomomasa Nakagawa wrote while on death row, according to a copy of the letter made available to Kyodo News through his lawyer in early March.

"I'm very very sure that they will fall into deep despair if they see the result of their attack," says the letter, which has no date or title.