A disinformation campaign claiming that the Covid-19 virus originated from an American military base in Maryland has gained popularity in China ahead of the release of a US intelligence report on the virus origins. In May, US president Joe Biden ordered a 90-day probe into whether the Covid-19 virus came from a lab accident or emerged from human contact with an infected animal. Until then, the "Wuhan lab leak" theory had been dismissed by most scientists as a fringe conspiracy theory. But now as the report is due to be released, China has gone on the offensive. In the past few weeks, Chinese sources have been amplifying a baseless claim that Covid-19 was made in the US. Using everything from rap music to fake Facebook posts, experts say the propaganda efforts have been successful at convincing the domestic Chinese audience to cast scepticism on international criticism of the country's role in the Covid-19 pandemic. But, experts say, it has done little to legitimise China to the outside world. What are the allegations? Most Americans may have never heard of Fort Detrick, but it is becoming a household name in China. Chinese propagandists have pushed a conspiracy suggesting that the Covid-19 coronavirus was made and leaked from the military installation in Frederick, Maryland, about 80 km (50 miles) north of Washington DC. Once the centre of the US biological weapons programme, it currently houses biomedical labs researching viruses including Ebola and smallpox. Its complicated history has sparked speculation in China. A rap song by the Chinese nationalist group CD Rev suggesting nefarious plots being hatched by the lab was recently endorsed by Zhao Lijian, the Chinese foreign ministry spokesman. The rhythms of the song -"How many plots came out of your lab/How many dead bodies hanging a tag/What are you hiding/Open the door to Fort Detrick" - are awkward, but its sentiment "speaks our mind," Mr Zhao wrote in a tweet in August. Mr Zhao, who is known for his aggressive style of diplomacy, has played an important role in spreading the "US origin" theory. Several tweets from his account last year first drew wide attention to Fort Detrick. "What's behind the closure of the biolab at Fort Detrick?" he wrote in July 2020, "When will US invite experts to investigate the origin of the virus in US?" In recent months, his calls have been joined by Chinese diplomats based in various countries, and the Chinese state broadcaster CCTV even aired an hour-long special report, "The Dark History behind Fort Detrick", focusing on breaches of containment at the lab in 2019, to bolster claims of lax lab security echoed by Chinese officials and state media. A related hashtag has had more than 100 million views on Weibo, China's Twitter equivalent. "We see a more sustained campaign involving more numerous and geographically spread accounts to promote the narrative," about Fort Detrick, says Ira Hubert, a senior investigative analyst at social analytics firm Graphika. Another popular theory, pushed by the nationalist tabloid the Global Times, attempts to connect the virus's origins to a US coronavirus expert, Dr Ralph Baric, and researchers at Fort Detrick. The newspaper suggested that Dr Baric created a new human-infecting coronavirus, citing a paper the North Carolina-based researcher co-authored about the virus's transmission from bats in Nature Medicine. In an editor's note, the journal said it was aware the paper was being used to spread the false theory, but the note was not included in the Global Times report. The newspaper also launched an online petition calling for Chinese netizens to sign an open letter demanding a World Health Organisation (WHO) investigation into Fort Detrick. People could "sign" the letter with a single click, and the appeal reportedly gathered more than 25 million "signatures".