Tomomitsu Niimi was born on March 9, 1964, in Japan. Not much is widely recorded about his early childhood or family life before he became entangled with Aum Shinrikyo, but like many others drawn into the cult, Niimi sought purpose, spiritual meaning, and belonging. By the late 1980s, he had become a close and trusted follower of Shoko Asahara, Aum Shinrikyo’s founder and self-proclaimed guru.

Asahara appointed Niimi to the high-ranking position of “Minister of Internal Affairs” within Aum. In this role, Niimi oversaw internal security and often acted as one of Asahara’s enforcers. He was deeply committed to the cult’s doctrines and displayed total loyalty to its leader. Niimi immersed himself in the group’s teachings, which mixed elements of Buddhism, Hinduism, apocalyptic Christianity, and Asahara’s authoritarian vision of the end times.

Niimi’s criminal activities with Aum Shinrikyo began in 1989. That year, Asahara ordered the elimination of lawyer Tsutsumi Sakamoto, who was preparing a lawsuit against the cult. Niimi was part of the group that strangled Sakamoto, his wife Satoko, and their baby son Tatsuhiko inside their home. The murders shocked Japan once their details were revealed years later, and Niimi’s role marked him as a man fully willing to carry out his leader’s deadly commands.

That same year, Niimi was also involved in the killing of Shuji Taguchi, a cult member who attempted to defect. Niimi personally took part in strangling Taguchi, demonstrating his willingness to enforce obedience through violence.

In the years that followed, Niimi rose in importance within Aum Shinrikyo. His loyalty earned him a spot among the cult’s inner circle. When the group developed chemical weapons, including sarin gas, Niimi was trusted with security operations. He once nearly died during a failed test run in which a sarin sprayer leaked, but he was saved when another member quickly administered an antidote.

On March 20, 1995, Niimi played a supporting role in the Tokyo subway sarin attack, one of the deadliest terror incidents in Japan’s history. During the coordinated attack, five Aum members punctured bags of liquid sarin with sharpened umbrellas on trains running through central Tokyo. Niimi acted as a driver, transporting Ikuo Hayashi, one of the perpetrators, to the site and aiding in the escape. The attack killed 13 people, severely injured more than 50, and caused temporary vision problems and respiratory issues for over 1,000 commuters.

Niimi was arrested in May 1995 along with many of Aum’s senior leadership. Unlike some other members, he refused to express remorse or turn against Asahara. He continued to voice support for his guru and showed no regret for his role in the murders and terror attacks.

After years of trials, Niimi was convicted of multiple murders and terrorism-related crimes. He was sentenced to death by hanging, and his conviction was upheld through appeals. On July 6, 2018, Niimi was executed at the Tokyo Detention Center, alongside Shoko Asahara and six other senior cult members.