Republicans in Congress want to subpoena a British-born scientist as part of an investigation in which they have concluded Covid-19 leaked from a Wuhan laboratory.
A report released by Republicans on the House foreign affairs committee said the "preponderance of evidence proves" the virus leaked from the Chinese research facility "sometime before September 12, 2019".
It cited "ample evidence" that the Wuhan Institute of Virology researchers – aided by US experts, and Chinese and US government funds – were working to modify coronaviruses to infect humans, and said such manipulation could be hidden.
The report foreshadowed the 90-day deadline on Aug 24 for a US intelligence review into the causes of the pandemic, which was ordered by Joe Biden, the US president.
It is believed US intelligence officials have not yet reached a conclusion on whether the virus escaped from the laboratory, or passed from animal to human in a nearby market.
Michael McCaul, the leading Republican congressman on the committee, accused China of "the greatest cover-up of all time".
In a statement accompanying the report, he said Wuhan scientists should be sanctioned. He also said British-born Dr Peter Daszak should be subpoenaed to answer "many questions" before the committee.
In June, Dr Daszak recused himself from an inquiry into the origins of the pandemic amid concern over his links to the Wuhan laboratory.
In the past, Dr Daszak has worked closely with Shi Zhengli, the Wuhan virologist sometimes nicknamed “Bat Woman” for her work on viruses in bats, and his US-based non-profit organisation had directed funding to Wuhan for virus research.
He was also a member of the World Health Organisation investigation into the source of the pandemic.
In February 2020, he co-authored a letter published in the Lancet which "strongly condemned conspiracy theories suggesting that Covid-19 does not have a natural origin".
China denies that a genetically modified coronavirus leaked from the facility in Wuhan, and also denies allegations of a cover-up.
China denies that a genetically modified coronavirus leaked from the facility in Wuhan. Wuhan Institute of Virology is pictured
The Republican report cited what it called under-reported information about laboratory safety protocols.
It detailed a request in July 2019 for a $1.5million overhaul of a hazardous waste treatment system, which was less than two years old. That request included maintenance on an "environmental air disinfection system".
It raised questions about how well such systems were working in the months leading up to the outbreak, the report said.
The report said: "Such a significant renovation so soon after the facility began operation appears unusual."
According to the report, satellite data in October showed a jump in visits to hospitals in Wuhan, along with a rise in people searching the internet for symptoms that could be linked to the virus.
It suggested the virus spread through Wuhan shortly before the Military World Games was held there in late October 2019.
In November, that event became an "international vector spreading the virus to multiple continents around the world" as athletes returned home, the report said.
Mr McCaul said: "It is our belief the virus leaked sometime in late August or early September 2019.
"When they realised what happened, Chinese Communist Party officials and scientists at the WIV began frantically covering up the leak, including taking their virus database offline in the middle of the night and requesting more than $1million for additional security.
"Therefore it is no longer appropriate for anyone to dismiss the notion this virus could have been genetically modified before it leaked from the WIV."