Katsuya Takahashi, a 56-year-old former member of the Aum Supreme Truth cult, denied being involved in unleashing sarin nerve gas on the underground rail system, in an attack that sparked rush hour chaos in the Japanese capital.
"I did not know the thing released was sarin," Takahashi, who has been charged with murder and other crimes, told Tokyo District Court.
"I didn't intend to kill people," he said, speaking in a quiet voice while dressed in a dark suit with a blue tie.
He is also accused of conspiring with other members to send an explosive to then-Tokyo governor Yukio Aoshima in 1995, an incident that injured a Tokyo government official.
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Takahashi, who was on the run for more than 17 years, was a one-time guard for Aum leader Shoko Asahara, and allegedly served as a driver when cult members released the gas.
Asahara, a partially blind guru who preached a blend of Buddhist and Hindu dogma mixed with apocalyptic messages, developed an obsession with sarin, becoming paranoid that his enemies would attack him with it.
During Takahashi's trial, several death-row inmates are scheduled to testify.
Takahashi, indicted after 192 other Aum members, may also face a death penalty if found guilty.
A verdict is expected to be delivered in late April.