Kim Jong-nam died in mysterious circumstances last week at an airport in the Malaysian capital Kuala Lumpur.
Police believe he was poisoned.
Malaysia has recalled its ambassador from the North Korean capital Pyongyang and has summoned the North Korean ambassador "to seek an explanation".
Malaysian police say they are now looking for four North Koreans.
Meanwhile a video which apparently shows CCTV footage of the attack on Kim Jong-nam has surfaced and aired on Japanese television.
On Friday, North Korean ambassador Kang Chol accused the government in Kuala Lumpur of colluding with "hostile forces", saying that Malaysia had "something to conceal".
South Korea has accused the North of orchestrating the incident, saying on Monday it was evidence of North Korean "terrorism getting bolder".
What has led to the diplomatic spat?
Malaysia was one of very few countries to maintain diplomatic relations with North Korea, but this killing has strained ties.
Malaysia has refused to accede to North Korean demands to release Kim's body into their custody without an autopsy.
That apparently prompted the comments on Friday by Pyongyang's ambassador to Malaysia - which provoked an angry response from the Malaysian foreign ministry.
It said his accusation was "baseless", adding that it was their responsibility to conduct an investigation as Kim had died on Malaysian soil.
What's happening to Kim's body?
Malaysian authorities are now waiting for the results of its autopsy. Kang Chol said his country would reject the result as it was done without the presence of its representatives.
Malaysia has also refused to release Kim's body, saying it needs to conduct DNA testing first.
Police are now seeking samples from family members. Kim is believed to have family living in Beijing and Macau.
Malaysian police have said that if there is no claim by next of kin and once they exhaust all avenues for DNA collection, they will hand the body over to the North Korean embassy.
Who has been arrested so far?
Kim is believed to have been attacked in the Kuala Lumpur airport departure hall on Monday by two women, using some form of chemical.
Fuji TV has aired grainy CCTV footage showing a man resembling Kim Jong-nam approached by a woman at the airport.
Another woman then quickly lunges from behind and wipes his face with a cloth. She is seen wearing a white top emblazoned with the letters "LOL".
The man is then seen seeking assistance from airport staff while gesturing at his face, and is escorted to a room.
Two women, one Indonesian and one Vietnamese, were among the first to be arrested. The Indonesian, named as Siti Aisyah, is said to have told Malaysian police she had been paid to perform what she thought was a prank.
The men have been named as Ri Ji Hyon, 33; Hong Song Hac, 34; O Jong Gil, 55, and Ri Jae Nam, 57.