She said: “What were the Beijing orders behind the riots? Why were those workers who died in the riots considered martyrs? I want to address these questions one by one.”

The 1967 riots, which killed 51 people – 15 in bomb attacks – and injured more than 800, were triggered by a labour dispute at an artificial flower factory in San Po Kong in April that year.

The row escalated quickly after the leftist camp and mainland officials stationed in the then British colony seized the opportunity to mobilise their followers to stage an anti-British struggle.

The documentary got its name because digging out information from the Hong Kong Public Records Office had been a disappointing process for Lo, who has worked for news departments at different TV stations for three decades. There were scant records in the archives.

So the two-hour documentary, which took four years to make, is based more on first-person accounts from rioters and police officers and others, records of the UK Foreign Office now held in the National Archive in London, and documents outside official channels.

One of the highlights is the notes of Ng Tik-chow, a senior communist leader in Hong Kong between 1950 and 1962, who worked under then Chinese premier Zhou Enlai on Hong Kong policies during the riots.

Lo managed to get Ng’s daughter, Ng Fai, who kept the original copy of her father’s notes from the time, to talk in front of the camera.

One page of the notes showed that state enterprise China Resources Company had hatched a dangerous plan, asking the ministry of foreign trade, “in order to potently support our struggle”, to “supply 700 dozen sugar cane swords as soon as possible”.

Ng, who learnt this from the ministry of foreign trade, put a note next to it: “I ordered [the swords] to be stopped right in Shenzhen”.

“Just imagine when 8,000 people are wielding [the sickles] in a highly agitated and hysterical state,” Ng’s daughter said in the documentary. “It is more detrimental than real bombs.”

The daughter, aged 15 in 1967, recalled her father saying he had also stopped a shipment of guns ordered by another state enterprise, China Merchants Group.

The director said: “In my opinion, Ng Tik-chow is a man who saved Hong Kong by halting two extreme left actions. He should be remembered.”

Ng was later purged on the mainland during the Cultural Revolution and his children were forced to work in villages and factories. The notes also touched on the role of Zhou, suggesting he had given instructions to the leftists, including approving a sum of HK$11.9 million from the Bank of China to keep up their struggle in Hong Kong.

But as that struggle was getting nowhere against the colonial government’s strong resistance, Zhou expressed Beijing’s official disapproval of the campaign at the end of 1967, and calmed the situation.

It was not easy for Lo to get funds for the project.

Her application to a seed fund open to documentaries on Hong Kong fell through. All possible funding channels in and outside the government she and her associates approached turned down the project. Lo had to rely on her own funds and some donations from the widow of a riot participant. If Lo can crowdfund enough cash, there may be screenings in schools and other community venues.

Lo will show a preview on Saturday, and hold a premiere at a university in March.

Lo has also faced harassment from unknown people on her filming journey. She recalled a few people coming up to snap photos right in her face when she stepped out of the building where she edited the clips in a subdivided flat.

Tapes of the documentary are now kept in three different places in Hong Kong, and one overseas, “to make sure the film will be seen”, she said.