It has been confirmed that North Korean leader Kim Jong-un's estranged half brother Kim Jong-nam was killed in Malaysia on Monday.

The incident, suspected by many as a political assassination to remove Kim Jong-un’s potential competitor, has raised concerns about the strengthened reign of terror by the young North Korean dictator.

The confirmation from the Malaysian government of Kim Jong-nam’s death came as late as Thursday, a delay partly attributed to a fake passport carried by him when he was killed at Kuala Lumpur International Airport on Monday morning. However, Seoul’s chief spy agency, the National Intelligence Service, said on Tuesday that it was sure the dead man is Kim Jong-nam.

Who killed him and in what manner still remains unconfirmed, but according to the intelligence sources and media reports so far, the murder suspects appeared to have sprayed an unidentified substance on his face.

BBC quoted a Malaysian police official as saying that a woman covered his face with a cloth laced with a liquid, although some initial reports said that Kim died after two women shot him with poisoned needles.

The oldest son of late former North Korean leader Kim Jong-il, Kim Jong-nam had been considered as the legitimate successor of the Kim dynasty until the late 1990s. However, he was known to have fallen out of his father’s favor due to his embarrassing attempt to illegally enter Japan to visit the Tokyo Disneyland in 2001.

The 45-year old, had been living outside the country since his younger brother rose to power in 2011.

Pyongyang’s alleged involvement in his death has yet been confirmed, but those who suspect foul play said his critical view of the third-generation power succession in the North and his close relationship with China might have prompted the North to kill him.

Some experts speculated the killing of Kim Jong-nam rather reveals the vulnerability of Kim Jong-un’s leadership, while others expected it will further cement his already well-established power.

Meanwhile, four people, including two women holding Vietnamese and Indonesian passports, have been arrested in relation to the case. A Malaysian male believed to be the boyfriend of the Indonesian suspect was arrested Friday. News agencies have reported that a fourth man arrested on Friday night, was holding a North Korean passport. The Malaysian police are still chasing several other male suspects.

On Saturday morning Malaysian police said they would conduct a second autopsy on Kim Jong-nam’s body, in the hopes of drawing a clearer conclusion about the cause of his death.