The new report said its findings show arbitrary executions and extrajudicial killings under state custody have continued under the rule of young leader Kim Jong Un despite worldwide criticism over how North Korea supposedly applies the death penalty without due judicial process.
The CIA declined to comment to The Journal.
The newspaper cited an unnamed "person knowledgeable about the matter" for the claim and said many details of Kim Jong Nam's relationship with the U.S. spy agency remained unclear.
"Several former USA 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 Journal said.
They also said Kim Jong-nam, who resided mainly in Macau, was nearly certainly in contact with security services of other countries, particularly China's. He was the eldest son in the current generation of North Korea's ruling.
According to Fifield, Kim Jong Nam usually met his handlers in Singapore and Malaysia, citing a source with knowledge of the intelligence. He had been living overseas for years but could have been seen as a threat to Kim Jong Un's rule.
But US intelligence agencies concluded that Kim Jong-nam was ill-suited to fill such a role, several former US officials said.
Writing in her book, The Great Successor: The Secret Rise and Rule of Kim Jong Un, she added: "Kim Jong-nam became an informant for the CIA, an agency with a track record of trying to bring down dictators it didn't like".
He died after being attacked at Kuala Lumpur International Airport on 13 February 2017.
Kim Jong Nam was previously reported to have met with a USA intelligence agent in a hotel in Malaysia, days before being killed, Japanese news outlet Asahi Shimbun reported in 2017.
The brothers remained estranged as adults, and in 2013 the younger Kim executed Chang Song-thaek, his aunt's husband, who was said to be close to Jong-nam.
A human rights group said Tuesday it has identified hundreds of spots where witnesses claim North Korea carried out public executions and extrajudicial state killings as part of an arbitrary and aggressive use of the death penalty that is meant to intimidate its citizens.
Kim Jong Un's half-brother was working as a Central Intelligence Agency informant before he was brazenly murdered in a Malaysian airport in 2017, according to a report Monday.
A report claims to have mapped the locations Kim's regime has held public executions.
The 30-year-old and her co-defendant, Indonesian Siti Aisyah, were charged with colluding with four North Koreans to murder Kim Jong Nam. But he fell from favour in the early 2000s.