The Journal said Kim Jong Nam met on several occasions with CIA operatives, but also that many details of his relationship with the agency remain unclear. "Kim Jong-nam provided information to them, usually meeting his handlers in Singapore or Malaysia", Washington Post journalist Anna Fifield wrote in The Great Successor: The Divinely Perfect Destiny for Brilliant Comrade Kim Jong-un. US and South Korean officials have blamed the attack on North Korea, which it denies.
The book says that security camera footage from Mr Kim's last trip to Malaysia showed him in a hotel elevator with an Asian-looking man who was reported to be a U.S. intelligence agent.
Malaysia's relations with North Korea have plummeted ever since the assassination of Kim Jong-nam (C) in February 2017.
Officials at South Korea's National Intelligence Service and Unification Ministry, which oversees ties with North Korea, said they couldn't confirm Monday's report by The Wall Street Journal, which attributed the details to an unidentified "person knowledgeable about the matter".
There was no official comment yet from Pyongyang on the allegation.
As the first-born son of North Korean leader Kim Jong Il who died in 2011, Kim Jong Nam was once thought to be next in line in the dynastic succession.
Trump hailed what he called a "beautiful" letter he received from Kim.
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Vinograd added that Trump seems to be sending a message to USA sources that "he's not going to say anything if they're assassinated with nerve agents".
Fifield says in her book that Kim Jong Nam met with intelligence agents in Singapore and Malaysia.
The two women - who have said they thought they were taking part in a harmless prank for a TV show - smeared the nerve agent on his face at the Kuala Lumpur International Airport on February 13, 2017. Both were released from jail in March and April 2019 after the murder charges were dropped.
It's unclear when Kim Jong Nam first met with Central Intelligence Agency officials or how often. He had a Portuguese and Chinese passport as well as two North Korean ones in different names, including Kim Chol.
Fifield said he ran gambling websites and "lived in the shadows amid gamblers, gangsters and spies", while keeping some links to the regime. The murder was part of an extended purge carried out by Kim Jong-un against potential rivals and their supporters, as he consolidated his power.
That, according to the Wall Street Journal report, would have limited his value as a source of intelligence.