US lawmakers concluded a two-year investigation Monday into the Covid-19 outbreak that killed 1.1 million Americans -- backing the theory that the virus likely leaked from a Chinese laboratory.

A 520-page report from the Republican-controlled House Select Subcommittee on the Coronavirus Pandemic looked at the federal and state-level response, as well as the pandemic's origins and vaccination efforts.

"This work will help the United States, and the world, predict the next pandemic, prepare for the next pandemic, protect ourselves from the next pandemic, and hopefully prevent the next pandemic," panel chairman Brad Wenstrup said in a letter to Congress.

US federal agencies, the World Health Organization and scientists across the planet have arrived at different conclusions about the most likely origin of Covid-19, and no consensus has emerged.

Most believe it to have spread from animals in China, but a US intelligence analysis said last year that the virus may have been genetically engineered and escaped from a virology lab in the Chinese city of Wuhan, where human cases first emerged.

The congressional panel was persuaded by the lab leak theory after meeting 25 times, conducting more than 30 transcribed interviews and reviewing more than one million pages of documents.

The investigation included two days of interviews behind closed doors with Anthony Fauci, the government scientist who became the nation's most trusted expert in the chaotic early days of the 2020 outbreak.

Fauci's clashes with former and incoming president Donald Trump over the response sparked fury on the right, and he now lives with security protection following death threats against his family.

Republicans accuse the 83-year-old immunologist of helping to set off the worst pandemic in a century by approving funding passed on to Chinese scientists they accuse of manufacturing the SARS-CoV-2 coronavirus that causes Covid-19.

Among its headline conclusions, the report said the National Institutes of Health had indeed funded contentious "gain-of-function" research -- which seeks to enhance viruses as a way of finding ways to combat them -- at the Wuhan Institute of Virology.

Fauci angrily denied covering up the origins of Covid-19 before the panel in June, arguing that it would be "molecularly impossible" for the bat viruses studied at the lab to be turned into the virus that caused the pandemic.

But the panel's report said SARS-CoV-2 "likely emerged because of a laboratory or research related accident."

The probe found that lockdowns "did more harm than good" and that mask mandates were "ineffective at controlling the spread of Covid-19," contradicting other research showing that masking in public does reduce transmission rates.

Social distancing guidelines also came under criticism, although travel restrictions were deemed to have saved lives.

Investigators found that Trump's Operation Warp Speed -- the publicly-funded project to develop Covid vaccines -- was a "tremendous success" but that school closures would have an "enduring impact" on US children.

© Agence France-Presse