A North Korean defector who was forced to return to the country after being detained in China has killed himself with poison while waiting to be interrogated by authorities on why he escaped.

The defector, believed to be in his 50s, was a nuclear scientist and researcher at the physics center of the State Academy of Sciences in Pyongyang.

He was part of a group of North Koreans detained in Shenyang city, China, on November 4 and forced to return to North Korea on November 17.

The North Korean defector who killed himself was a nuclear scientist and researcher at the physics center of the State Academy of Sciences in Pyongyang. Pictured above, Kim Jong-Un looks at a metal casing with several men at an undisclosed location earlier this year

The defector killed himself hours after he was put in solitary confinement in the State Security Department in Sinuiju city in North Korea - just across the border from China.

'He died before he could be questioned about the reasons for his escape, who had helped him, and what his route had been,' an insider told RFA.

The insider said the defector was likely searched several times while being taken from China to Sinuiju, so it remains 'a mystery' how he concealed the poison he took.

Before fleeing to China, the defector had taken leave from his research job 'because he was showing signs of anxiety over his research projects', the insider said.

Officials are now investigating how the man crossed the Yalu River from Sinuiju in to China and when he met up with other defectors. Pictured above, North Korean soldiers patrol along the bank of the Yalu river in Sinuiju in 2013

The defector killed himself hours after he was put in solitary confinement in the State Security Department in Sinuiju city in North Korea - just across the border from China. Pictured above, Kim Jong-un speaks at a conference earlier this month in Pyongyang

'Suddenly, he visited relatives near the border without letting his family know and without carrying valid documents for travel,' the insider told RFA. 'And when he learned that the authorities were looking for him, he simply disappeared.'

The had previously been named by North Korean insiders as Hyun Cheol Huh, but it is unclear if that is his real name, as officials sometimes use fake names to refer to people in custody.

Officials are now investigating how the man crossed the Yalu River into China and when he met up with other defectors.

The man had reportedly not revealed to Chinese officials that he was a nuclear scientist.