Assailants used a deadly chemical weapon called VX nerve agent to kill the estranged half-brother of North Korean leader Kim Jong-un, said the Malaysian Police on Friday. Swabs taken from the eye and the face of Kim Jong-nam revealed the use of VX nerve agent.

Investigators are now trying to find out how the banned substance, which is considered the most toxic nerve agent in the world and features on a United Nations’ list of mass destruction weapons, was brought into the country. They are screening the airport and other locations to look for radioactive material. “If the amount of the chemical brought in was small, it would be difficult for us to detect,” said police chief Khalid Abu Bakar, according to Reuters. However, an employee of Malaysia Airports Holdings, which manages the airport, told The Guardian it had not closed the terminal or been contacted by police about any decontamination.

Kim Jong-nam was assassinated at Kuala Lumpur International Airport on February 13 when two women attacked him with “poisoned needles” before fleeing the site. The police chief said that although both the women had washed their hands before they fled the airport, one of them was suffering from its side effects.

The VX nerve agent neither has any taste nor any odour. It can be manufactured as a liquid, cream or aerosol. But what makes it the most toxic substance is the fact that it can prove fatal within 15 minutes when absorbed in large amounts.

North Korea is believed to be world’s third-largest chemical weapons hoarder. In 2014, South Korean had accused the North of stocking up 2,500 to 5,000 tons of chemical weapons and said that it had a capacity to produce a variety of biological weapons.

The assassination has already strained bilateral relations and triggered diplomatic tension. On February 19, Malaysia had recalled its envoy to Pyongyang and summoned North Korea’s envoy, Kang Chol, for saying Kuala Lumpur authorities were colluding with “external forces” in the handling of the investigation. With the new findings, the stakes over how Malaysia and the international community will respond are very high now, according to The New York Times.

The Malaysian Police have, till date, arrested one North Korean man, a Vietnamese woman, an Indonesian woman, and a Malaysian man. They had also identified two more suspects – one a senior official in the North Korean embassy and the other related to the North Korean airline.

Kim Jong-nam, the eldest son of late dictator Kim Jong-il, had challenged his half brother’s succession to the top post. Officials said he had been trying to take over the administration. He had been living in Macau under Chinese protection after a reported dispute with his father over his attempt to enter Japan with a fake passport, South Korea’s intelligence agency told its parliament.