In Japan, the death penalty is carried out by hanging. The prisoner is only notified about an hour beforehand, and family members only find out after the fact.

So Rika Matsumoto knew that any day, her father could be gone.

Her father was Shoko Asahara, the leader of Aum Shinrikyo, a Japanese doomsday cult. On March 20, 1995, Aum members spread sarin gas in the Tokyo subway system during morning rush hour. 17 people died, and thousands more fell ill. Asahara was arrested, along with several other cult members. Rika was 12 years old at the time.

Asahara was executed on July 6, 23 years after the attacks.

A few years ago, Matsumoto had begun to publicly campaign for a retrial for her father, saying he'd been mentally unfit to stand trial.

But even now that her father is dead, some people are still asking questions about what happened.

To see more about how Japan is still dealing with the deadliest attack on Japanese soil since WWII, watch the video above.

This segment originally aired July 9, 2018 on VICE News Tonight on HBO.