Marcus Detrez landed in Beijing last month with a leather briefcase that he said contained hundreds of his grandfather's pictures from the conflict, which ended in 1945 after widespread atrocities in China.

State-run media outlets such as China Daily and CCTV reported that the 26-year-old found the yellowed images while rummaging through the garage of his family home in 2021.

"All of them were taken by my grandfather Roger-Pierre Laurens in Shanghai," says text over a video featuring Detrez and his companions on Douyin, China's version of TikTok.

Detrez's claims spread quickly, racking up tens of thousands of shares on Douyin ahead of the 80th anniversary of the end of World War II later this year.

However, an AFP digital investigation found many of the pictures were taken by other people.

Two photos have been attributed to the Associated Press, while some digital copies of the images were published years ago by Chinese media outlets.

Jamie Carstairs, former manager of the Historical Photographs of China (HPC) project at Britain's University of Bristol, said that Detrez should be "congratulated for his kind donation" but that "care should be taken".

"It is not correct to say that the photos were purportedly taken by Roger-Pierre Laurens," Carstairs said. "Some of them might have been, but others were not."

- 'Return the truth' -

Japan's early 20th-century imperial ambitions resulted in military occupations across large parts of Asia, including China.

After invading in the 1930s, Japanese soldiers committed atrocities like the 1937 Nanjing Massacre, a six-week spree of mass murder, rape and looting that killed tens if not hundreds of thousands of the city's inhabitants.

Detrez said in a May 2024 Douyin video that his grandfather "came to Shanghai in 1930 as an entrepreneur" and "took these photos fearlessly" despite two of his sons being killed by the Japanese.

He later told a Beijing broadcaster he wanted to "return the truth to the Chinese".

After state broadcaster CGTN reported Detrez "expressed an intention to donate them to Chinese institutions", a Shanghai Sino-Japanese War memorial hall told CCTV that his photos had been received and were "pending professional appraisal".

- 'Duplicate prints' -

However, AFP found several inconsistencies.

A photo featuring a Japanese naval parade through a Shanghai street traces back to online archives from the US Naval History and Heritage Command, which told AFP it was taken by a chief warrant officer in 1937.

Carstairs said the HPC database, which includes a large collection of original materials and digitised versions of historic images, indicates a few of the pictures appear in an album from Chinese photographer Ah Fong, who was active in the 1930s.

Most of those images were taken between August and November 1937 by two photographers, identified only as "S.S." and "S.C.S".

"Copies of the album of photos sold by Ah Fong come up for sale from time to time," Carstairs told AFP, adding that Detrez appears to have "duplicate prints of some photos".

Carstairs said that while the source of historic images is "often quite difficult to find out", it is "easy to find out who compiled albums or collected photographic prints".

Despite the mostly positive reaction in China, some social media users questioned Detrez's claims.

"The French guy and his companions... used these photos to steal traffic and engagement, thinking all Chinese are fools," one WeChat user wrote in a post.

Detrez did not respond to an AFP request for comment, but on Douyin called challenges to the authenticity of the photographs "malicious speculation".