Tomomasa Nakagawa, a former senior member of the doomsday cult Aum Shinrikyo who is now on death row, has published a research paper on the deadly VX nerve agent, it was learned Wednesday.

The paper on VX was written jointly with Anthony Tu, honorary professor at Colorado State University. It was published Monday in a digital edition of the Japanese journal Forensic Toxicology.

VX was used in the 2017 assassination in Malaysia of Kim Jong Nam, the exiled half brother of North Korean leader Kim Jong Un.

Assailants are believed to have smeared two less-toxic substances on the victim’s face and mixed them in order to generate VX, the paper said.

That method is different from a technique to produce VX by mixing substances that was developed by the U.S. Army, according to the paper.

Pointing to three VX attacks committed by Aum in Japan between 1994 and 1995, the paper warned that the possibility that toxic nerve agents such as VX will be used in future terror attacks must not be ignored.

According to Tu, Nakagawa decided to write the paper in English, hoping it will also be read abroad.

He was delighted when he learned that the paper was going to be published in the journal, Tu said.

Nakagawa, a former doctor, was sentenced to death in 2011 for his involvement in VX and sarin attacks, including the 1995 sarin attack on Tokyo’s subway system.

In March, he was transferred from the Tokyo Detention House to the Hiroshima Detention House.