Malaysian police say the chubby 45-year-old was jumped by two women who squirted some kind of liquid in his face.
Jong-Nam told staff he was suffering from a headache and was taken to the airport clinic grimacing in pain, according to Malaysian media citing CCTV footage from the airport.
One of the women walked to a taxi rank immediately after the attack, according to the same footage.
He was rushed to hospital suffering from a seizure but was dead before he arrived.
South Korea has pointed the finger of blame at the North, citing a “standing order” from Jong-Un to kill his sibling and a failed assassination bid in 2012 after he criticised the regime.
– Woman in ‘LOL’ top –
Pyongyang has made no comment on the killing, and there has been no mention of it in North Korean media.
AFP correspondents in Pyongyang say celebrations to mark the birthday of Kim Jong-Il, the late father of both men, have gone ahead without reference to the death.
Malaysian police on Wednesday arrested a 28-year-old woman carrying a Vietnamese passport which identified her as Doan Thi Huong.
Local media said she was the woman seen in CCTV images from the airport wearing a white top with the letters “LOL” emblazoned on the front.
Officers later arrested Muhammad Farid Bin Jalaluddin, a 26-year-old Malaysian man. He led them to his girlfriend, a 25-year-old Indonesian named Siti Aishah according to her travel documents.
Jakarta confirmed late Thursday that Aishah was an Indonesian citizen and embassy officials were providing her with legal assistance.
Selangor state police chief Abdul Samah earlier told AFP he was looking for several more suspects, but declined to say how many were being sought.
First-born Jong-Nam was once thought to be the natural successor to his father, but on Kim Jong-Il’s death in 2011 the succession went to Jong-Un, who was born to the former leader’s third wife.
Reports of purges and executions have emerged from the current regime as Jong-Un tries to strengthen his grip on power in the face of international pressure over nuclear and missile programmes.
The most notable of these was the 2013 execution for treason of the young leader’s influential uncle, Jang Song-Thaek.