VX nerve agent, a chemical the United Nations classifies as a weapon of mass destruction, was used to kill the estranged half-brother of North Korean leader Kim Jong Un in a bizarre murder in Malaysia last week, police said on Friday.

Kim Jong Nam was killed on Feb. 13, shortly after being assaulted at the airport in the Malaysian capital, Kuala Lumpur, by two women who wiped the chemical on his face as he prepared to board a flight to the Chinese territory of Macau.

South Korean and U.S. officials have said they believe North Korean agents assassinated Kim Jong Nam. He had been living with his family in Macau under Beijing’s protection and had spoken out against the North Korean regime.

Malaysian police were investigating whether the VX – which is believed to be the most toxic known nerve agent and is banned globally except for research – was brought into the country or made there.

“If the amount of the chemical brought in was small, it would be difficult for us to detect,” police chief Khalid Abu Bakar told reporters.

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