Malaysian police say VX nerve agent, a chemical on a UN list of weapons of mass destruction, was used to kill the estranged half-brother of North Korean leader Kim Jong Un.
Kim Jong Nam died on February 13 after being assaulted by two women who wiped the toxic chemical on his face as he prepared to board a flight to Macau at Kuala Lumpur International Airport.
Malaysian police were investigating whether the illegal VX - believed to be the most toxic known nerve agent - was brought into the country of made there, and authorities were sweeping the airport and other locations for radioactive material.
"If the amount of the chemical brought in was small, it would be difficult for us to detect," Khalid Abu Bakar told reporters.
"(We are looking to) sweep all locations that we knew the suspects went to. We will get the experts from the atomic energy department to go to the location and sweep it to see if radioactive (material) is still there."
Police arrested the two women - one Vietnamese and the other Indonesian - and a North Korean man last week. They are also seeking seven other North Koreans wanted in connection with the case, including a diplomat at the embassy in Kuala Lumpur.
South Korean and US officials have said they believe North Korean agents assassinated Kim Jong Nam, who had been living in the Chinese territory of Macau under Beijing's protection.
North Korea is believed to have the world's third-largest stockpile of chemical weapons, according to the Nuclear Threat Initiative project, which analyses weapons of mass destruction.
South Korean analysts have identified sarin and VX as the focus of the North Korean chemical weapons program.
Police said swabs taken from the eye and the face of the North Korean murder victim revealed the presence of VX nerve agent, or S-2 Diisoprophylaminoethyl methylphosphonothiolate.
"Other exhibits are under analysis," Khalid said in a statement that cited findings from a preliminary report from the government's chemical weapons analysis unit.
Khalid said the two women who were paid to carry out the assault had washed their hands before fleeing from the airport.
But he said one of them was suffering from the effects of the chemical and was vomiting.
Airport camera footage released on Monday by Japanese broadcaster Fuji TV shows the moment they assaulted Kim Jong Nam. In later clips he is seen asking airport officials for medical help, and rubbing his eyes and stumbling as he entered an airport clinic. He died on the way to hospital.
Malaysian authorities have requested Interpol to put an alert out to apprehend four North Korean suspects who are believed to have fled Malaysia on the day of the attack.