Malaysian police are investigating how the highly toxic nerve agent VX, identified as the chemical used in the murder of Kim Jong-nam, the estranged half brother of North Korean Supreme Leader Kim Jong-un got into the country.

Police chief Inspector-General of Police Khalid Abu Bakar told reporters: "We don't know yet [about the origins of the chemical]. We are trying to find out."

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"But if it is a very small amount, it would have been very hard to detect," he added, according to The Star.

Reuters reported that police are investigating whether the chemical, believed to be the most toxic nerve agent in the world, was brought into Malaysia or made in the country itself.

Jong-nam died after being poisoned by the chemical at the Kuala Lumpur International Airport 2 on 13 February.

According to the US Army's Edgewood Chemical Biological Center, the chemical is fatal after only 15 minutes if absorbed in large doses.

The news agency said that in 2015, traces of sarin and the VX nerve agent were found at a military research site in Syria, that had not been declared to the global chemical weapons watchdog.

Separately, Khalid told reporters that one of the two women held in custody over the killing of Jong-nam also suffered the effects of the VX nerve agent.

"She was vomiting," he said, in reference to the symptoms the woman suffered from.

He declined to elaborate, Reuters reported. The police chief also did not identify which of the two female suspects was affected.

Malaysian police nabbed four people — Indonesian woman Siti Aisyah, a Vietnamese woman Doan Thi Huong and two other men, a Malaysian and North Korean — over the murder of Jong-nam.

They have on Thursday (23 February) requested Interpol to put an alert out for additional North Korean suspects who they believe have fled the country on the day of the attack.

The ppolice are also seeking to question the second secretary at the North Korean embassy in Kuala Lumpur and two others North Koreans, including an employee of the state-owned airline Air Koryo.