The source says then-Chinese president Hu Jintao met with Jang Song Thaek, a high-ranking North Korean official, in August 2012, just months after the death of Kim Jong Il. Jang was Kim Jong Un's uncle and, according to the source, he told Hu he wanted Kim's half-brother, Kim Jong Nam, to be the country's new leader.
The source says the meeting was secretly taped by Zhou Yongkang, a high-ranking Chinese Communist Party official, who allegedly tipped off Kim Jong Un.
Months later, Jang was executed for treason. And several years later, Kim Jong Nam was assassinated, fatally poisoned by a highly toxic nerve agent at Kuala Lumpur International Airport.
The source says North Korea began to distrust the Chinese leadership after learning they had done nothing to prevent what it saw as a move to overthrow its government.
On the other hand, Beijing also grew suspicious of Pyongyang for purging Jang, who had previously acted as an intermediary between the countries.
China and North Korea have historically been close, but their leaders have not met since Kim Jong Un took power.
Expert suspects the incident is behind worsening ties
Nanzan University Professor Shunji Hiraiwa says Jang may have thought Kim Jong Nam would be easier to deal with than Kim Jong Un. He says Jang had too much confidence in his power as mentor to Kim Jong Un.
Hiraiwa also says that the tip-off may have undermined the countries' mutual trust, resulting in the current state of strain.
He also says the mood of distrust has set the two countries further apart over North Korea's nuclear and missile programs.
Close to former Chinese President Jiang Zemin, Zhou joined the country's top leadership under Hu Jintao. As a security chief, he is said to have wielded great influence.
The 75-year-old hails from Jiangsu Province. He held a senior post in the oil ministry and took the helm of a state-owned oil and natural gas firm in the 80s and 90s.
In 1999, he became the Communist Party chief of Sichuan Province, and three years later rose to the top post at the Public Security Ministry.
In 2007, he joined the party's Politburo Standing Committee. Despite being the lowest-ranked of the nine members, he is said to have had immense power as the head of a committee overseeing public security departments, intelligence agencies, prosecutors and courts.
Zhou is believed to have been a backer of Bo Xilai, the former Communist Party chief of Chongqing City. Bo was jailed for corruption and abuse of power, and allegations surfaced that he conspired with Zhou to topple incumbent President Xi Jinping and other leaders.
Zhou was one of those detained amid Xi's anti-corruption campaign. In 2015, a court sentenced him to life in prison for taking bribes and leaking state secrets.
He became the first former member of China's top leadership to be found guilty of corruption.
The ruling was broadcast live on state television. Zhou's hair had turned completely white in the time since his arrest.
Jang Song Thaek
Jang is believed to have been mentor to Kim Jong Un after he succeeded his father as leader of North Korea.
He was born in North Hamgyong Province in 1946 and later married Kim Jong Il's sister. After graduating from Kim Il Sung University, Jang joined the ruling Workers' Party. Thanks to his ties with Kim, Jang began climbing the party ladder and secured a senior post at the key Organizational Guidance Department.
In 2010, he was appointed vice chairman of the National Defense Commission. At Kim Jong Il's funeral a year later, he accompanied a hearse carrying the body, together with Kim Jong Un.
Jang worked to boost economic cooperation with China and played a leading role in economic reforms and attracting foreign investment. During his 2012 visit to Beijing, he held talks with then-president Hu.
But in 2013, he was suddenly dismissed from all posts and the Workers' Party for allegedly engaging in "anti-party and counter-revolutionary acts." He was executed immediately after being sentenced to death by a special military tribunal. The court said he had committed a heinous crime in an attempt to overthrow the state.