Kim Jong Nam, 45, the older half-brother of North Korean leader Kim Jong Un, was found dead at Kuala Lumpur Airport, Malaysian police said Tuesday in what may have been an assassination carried out by North Korean agents.

Malaysian Criminal Investigation Department Director Mohmad Salleh confirmed to local news outlets that Kim Jong Nam died Monday at the airport after becoming sick before boarding a flight. A cause of death has not been determined.

South Korea media reports, citing unnamed government sources, said he had been jabbed with poisoned needles by two women at the airport in Kuala Lumpur. The women are suspected to be North Korean agents and remained at large after fleeing the scene in a cab, according to the reports.

Kim Jong Nam was the eldest son of North Korean leader Kim Jong Il, and was at one time considered the heir apparent to rule the isolated country, which has been governed by three generations of the Kim family.

In 2001, he was arrested at Tokyo’s Narita Airport after trying to enter Japan on a forged passport from the Dominican Republic. He told police he wanted to visit Tokyo Disneyland.

After falling out of favor with his father, Kim Jong Nam lived in exile in Macau, a Chinese island known as a gambling mecca. In emails to the Japanese newspaper Tokyo Shimbun, he said the rift had grown because he insisted on reforms. "After I went back to North Korea following my education in Switzerland, I grew further apart from my father because I insisted on reform and market-opening and was eventually viewed with suspicion,” he wrote.

Kim Jong Nam also criticized North Korea’s dynastic succession and that he had no interest in running the country, which remains in the iron grip of his half-brother. It was widely speculated that he lived in fear for his life under Kim Jong Un's harsh regime, and there have been reports of other assassination attempts in the past.

North and South Korean authorities did not immediately comment on reports. If confirmed, it would be the highest-profile North Korean death since Kim Jong Un's uncle, Jang Song Thaek, was executed in December 2013.