A whistleblower who says he was removed from his government post for raising concerns about coronavirus preparedness told a congressional hearing on Thursday that the United States could face “the darkest winter” of recent times if it does not improve its response to the pandemic.

Hours after President Donald Trump railed against him on Twitter, whistleblower Rick Bright testified to a US House of Representatives panel about readiness for the outbreak.

Bright was removed last month as director of the Biomedical Advanced Research and Development Authority, or BARDA, a part of the US Department of Health and Human Services responsible for developing drugs to fight the coronavirus.

“What we do must be done carefully with guidance from the best scientific minds. Our window of opportunity is closing. If we fail to improve our response now, based on science, I fear the pandemic will get worse and be prolonged,” Bright said during his testimony.

Bright testified to the subcommittee on health that he would “never forget” an e-mail he got in January from a US supplier of medical-grade face masks warning of a dire shortage.

“He said ‘we are in deep s**t. The world is. We need to act,’” Bright said. “And I pushed that forward to the highest level that I could of HHS and got no response.”

Bright testified that the US still lacked a comprehensive plan for ensuring a supply of basic supplies like swabs needed to administer coronavirus tests.

Trump, who has been pushing for the US economy to reopen quickly, dismissed Bright as a “disgruntled employee” on Twitter on Thursday morning before the hearing begun.

Later on Thursday, Trump told reporters at the White House that he had watched some of Bright’s hearing.

“To me he’s nothing more than a really disgruntled, unhappy person,” Trump said, adding that he did not know Bright.

“Everything he’s complaining about was achieved,” HHS Secretary Alex Azar told reporters.

Earlier this week, leading US infectious disease expert Dr. Anthony Fauci warned a Senate committee that a premature lifting of lockdowns could lead to additional outbreaks of the deadly coronavirus.

Trump on Wednesday described Fauci’s warning as unacceptable.