 -- Kim Jong Nam, the estranged half brother of North Korean leader Kim Jong Un, had been exposed to nerve agent, police in Malaysia said Thursday.

According to the police, a preliminary analysis found VX nerve agent on the face of the victim, who was killed on Feb. 13.

The Centers for Disease Control says VX "is a human-made chemical warfare agent classified as a nerve agent."

"Nerve agents are the most toxic and rapidly acting of the known chemical warfare agents," the site adds.

According to The Associated Press, two women suspected of poisoning Kim Jong Nam were trained to deliver toxic substances by putting them on their hands and wiping them on his face.

"We strongly believe it is a planned thing and that they have been trained to do that. This is not just like shooting a movie," said Inspector-General of Police Khalid Abu Bakar, according to the AP.

As of Tuesday, four people had been arrested, including the two women. At least one claimed she was tricked into participating in the attack, the AP said.