A North Korean envoy has rejected a Malaysian post-mortem examination finding that the VX nerve agent killed Kim Jong Nam, saying he probably died of a heart attack.
The death of Kim Jong Nam, the estranged half-brother of North Korea's ruler, has unleashed a diplomatic battle between Malaysia and North Korea.
The post-mortem examination is especially sensitive because North Korea had asked Malaysia not to perform one, but authorities carried it out anyway, saying they were following the law.
Amid growing fallout from the killing, Malaysia announced it is scrapping visa-free entry for North Koreans.
Malaysian officials say two women smeared VX nerve agent - a banned chemical weapon - on Kim Jong Nam's face as he waited for a flight at Kuala Lumpur's airport on February 13.
Kim Jong Nam died within 20 minutes, authorities say. No bystanders reported falling ill.
The women, who were caught on grainy surveillance video, have been charged with murder.
Both say they were duped into thinking they were playing a harmless prank and did not know they were handling a lethal toxin.
Malaysia's post-mortem exam finding that VX nerve agent killed Kim Jong Nam boosted speculation that North Korea orchestrated the attack.
Experts say the oily poison was almost certainly produced in a sophisticated state weapons laboratory.
North Korea has denied any role and accused Malaysia of bias.
Ri Tong Il, the former North Korean deputy ambassador to the United Nations, told a news conference that it made no sense to say the two women used such a deadly toxin without also killing or sickening themselves and people around them.
Mr Ri said Kim Jong Nam had a history of heart problems and had been in hospital in the past.
He said he understood that Malaysian officials found medication for diabetes, heart problems and high blood pressure in Kim Jong Nam's belongings and concluded he was not fit to travel.
"This is a strong indication that the cause of death is a heart attack," Mr Ri said.
Malaysian police said the attackers knew what they were doing and had been trained to go immediately to the bathroom and wash their hands.
Police cannot confirm whether the two women may have been given antidotes before the attack.
An antidote, atropine, can be injected after exposure and is carried by medics in war zones where weapons of mass destruction are suspected.
North Korea does not acknowledge that it was Kim Jong Nam who died.
Instead, it refers to the victim as Kim Chol, the name on the diplomatic passport he was carrying.
Malaysia has confirmed that the victim was Kim Jong Nam.
The case has badly frayed once-warm ties between Malaysia and North Korea.
While it is not one of North Korea's key diplomatic partners, Malaysia has been one of the few places in the world where North Koreans could travel without a visa.
As a result, for years it has been a quiet destination for North Koreans looking for jobs, schools and business deals.
That could all change after Kim Jong Nam's death.
Deputy prime minister Ahmad Zahid Hamidi said the visa-free arrangement with North Korea will be scrapped starting on Monday for national security reasons.
He also criticised the North Korean ambassador in Kuala Lumpur, who has accused Malaysia of "trying to conceal something" and "colluding with hostile forces".
"We don't want to make enemies, but if they had used Malaysia for their own agenda, they should not accuse Malaysia and tarnish our image on the international stage," Mr Zahid said.
"We will act firmly to guarantee the safety of our people. Don't ever use Malaysia as a base to do anything you like."
Earlier, Malaysian authorities said they will release a North Korean man, Ri Jong Chol, from custody on Friday because of a lack of evidence.
Officials never said why they arrested him.
But Malaysian Attorney General Mohamad Apandi Ali said he will be deported because he does not have valid travel documents.
Custody of Kim Jong Nam's body has become a point of contention between the two countries.
Malaysia has resisted giving up the body without getting DNA samples and confirmation from next of kin.
Kim Jong Nam is believed to have two sons and a daughter with two women living in Beijing and Macau.
North Korea has unsuccessfully demanded that Malaysia hand the body over.
Malaysian authorities are seeking seven other North Korean suspects, four of whom fled the country on the day of Kim Jong Nam's death and are believed to be back in North Korea.
Others sought include the second secretary of North Korea's embassy and an employee of North Korea's state-owned airline, Air Koryo.
Kim Jong Nam was estranged from his half-brother, North Korean leader Kim Jong Un.
He reportedly fell out of favour with their father, the late Kim Jong Il, in 2001, when he was caught trying to enter Japan on a false passport to visit Tokyo Disneyland.
Isolated North Korea has a long history of ordering killings of people it views as threats to its regime.
Kim Jong Nam was not known to be seeking political power, but his position as eldest son of the family that has ruled North Korea since it was founded could have made him appear to be a danger.