Kim Jong Nam died after his face was smeared with the nerve agent VX, a banned chemical weapon, in February 2017.
He had been at Kuala Lumpur's airport at the time.
An unnamed "person knowledgeable about the matter" told the Wall Street Journal that there had been a "nexus between the CIA and Kim Jong Nam".
Former officials also said Mr Kim had likely been in contact with the security services of other countries, particularly China's.
"Several former US officials said the half-brother, who had lived outside of North Korea for many years and had no known power base in Pyongyang, was unlikely to be able to provide details of the secretive country's inner workings," the newspaper added.
The CIA has declined to comment on the report.
A book due to be published on Tuesday says Mr Kim would usually meet his handlers in Singapore or Malaysia.
Author and Washington Post reporter Anna Fifield quotes a source with knowledge of intelligence in her book The Great Successor.
According to her, security camera footage showed Mr Kim in a hotel with a man reported to be a US intelligence agent.
His backpack also contained $120,000 (£94,600) in cash which could have been used as payment for his intelligence-related activities.
Kim Jong Nam, the eldest son of late North Korean leader Kim Jong Il, had spoken out publicly against his family's dynastic control of the isolated nation.
He had been living in the Chinese territory of Macau under China's protection.
The 45-year-old was overlooked to succeed his father after a botched attempt to enter Japan in May 2001 on a forged passport.
He claimed he wanted to visit Disney's Tokyo resort, but the incident embarrassed his father and Kim Jong Un was selected for the top job instead.