Republicans in the US House of Representatives on Tuesday held a forum of experts discussing the theory that a leak at a Chinese virology lab is responsible for the coronavirus pandemic as they urged Democratic lawmakers to fully investigate the virus’ origins amid renewed interest in the theory often labeled a conspiracy.

The Republicans of House committees on oversight and the coronavirus, adamant that the Wuhan lab is the origins of the pandemic, accused House Speaker Nancy Pelosi and other Democrats of stonewalling GOP attempts to take up resolutions or answer their requests for a full probe.

“Speaker Pelosi, this is not a distraction,” Rep. Steve Scalise, ranking member of the subcommittee, said during his opening remarks to the forum. “This is exponentially worse than Chernobyl, and the American people are demanding answers and deserve the truth.”

The theory that the virus escaped from a lab in Wuhan, ground zero of the coronavirus pandemic, has grown in prevalence in recent months.

In May, 18 scientists published a letter in the journal Science calling for an investigation into the Wuhan theory as there wasn’t enough evidence to rule it out as a possibility.

Scalise has sent two letters – one in late May and the other in early June – requesting the House oversight and reform committee as well as the select committee on the coronavirus to hold panels on the theory. Tuesday’s forum was organized after those requests were rehected June 11 by the committees’ Democratic leaders.

The House has also yet to take up a bill the Senate passed late last month to direct the National Intelligence Director Avril Haines to declassify all information about potential links between the Wuhan Institute of Virology and the origins of the coronavirus pandemic.

“The evidence continues to mount that this was a manmade disaster that started in the Wuhan lab,” Scalise said. “If that is the case, then it might be the crime of the century – exponentially worse than Chernobyl.”

The Republicans pointed to instances of China withholding information about its initial outbreak and accusing it of lying about whether the virus could be transferred from person to person as Beijing’s efforts to conceal the severity of its outbreak, which they said was aided by the World Health Organization, a criticism of the U.N. health body that former President Donald Trump often repeated and used in his attempt to pull the United States from it while in office.

Rep. James Comer, ranking member of the House oversight and reform committee, also accused the World Health Organization of aiding China in its alleged cover-up by producing a “one-sided report” on the pandemic’s origins in April.