In 1994 and 1995, the Japanese cult Aum Shinrikyo used homemade VX to attack three people, one of whom died.

North Korea is estimated to have a chemical weapons production capability of up to 4,500 metric tons during a typical year and 12,000 tons during a period of extended crisis. It is widely reported to possess a large arsenal of chemical weapons, including mustard, phosgene and sarin gas, a United States Congressional Research Service report said last year.

The announcement by Malaysia’s police chief came just a day after North Korea denied any responsibility for Mr. Kim’s death, accusing the Malaysian authorities of fabricating evidence of Pyongyang’s involvement under the influence of South Korea.

With the North’s reclusive government on the defensive about the Feb. 13 killing of Mr. Kim, the estranged half brother of Kim Jong-un, at the airport for the Malaysian capital, Kuala Lumpur, a statement attributed to the North Korean Jurists Committee said the greatest share of responsibility for the death “rests with the government of Malaysia” because Kim Jong-nam died there. And in what could be seen as a threat to Malaysia, the statement noted that North Korea is a “nuclear weapons state.”

But in a case that has been filled with mysteries and odd plot twists, North Korea still would not acknowledge that the man killed was indeed Kim Jong-nam. And it gave no indication that it would agree to Malaysia’s demands to question a senior staff member at the North Korean Embassy in Kuala Lumpur in the investigation into Mr. Kim’s death.

Relatives and acquaintances of the two women Malaysia has accused of carrying out the killing, by applying poison to Kim-Jong-nam’s face as North Korean agents looked on, insisted they must have been duped into doing so, though the Malaysian authorities say otherwise.

“I don’t believe Huong did such a thing,” said Doan Van Thanh, father of Doan Thi Huong, 28, a Vietnamese woman being held in Malaysia. “She was a very timid girl. When she saw a rat or frog, she would scream.”