WASHINGTON—Early on, the dozen federal officials charged with defending America against the coronavirus gathered day after day in the White House Situation Room, consumed by crises. They grappled with how to evacuate the United States consulate in Wuhan, China; ban Chinese travellers; and extract Americans from the Diamond Princess and other cruise ships.

The members of the coronavirus task force typically devoted only five or 10 minutes, often at the end of contentious meetings, to talk about testing, several participants recalled. The U.S. Centers for Disease Control and Prevention (CDC), its leaders assured the others, had developed a diagnostic model that would be rolled out quickly as a first step.

But as the deadly virus from China spread with ferocity across the U.S. between late January and early March, large-scale testing of people who might have been infected did not happen — because of technical flaws, regulatory hurdles, business-as-usual bureaucracies and lack of leadership at multiple levels, according to interviews with more than 50 current and former public-health officials, administration officials, senior scientists and company executives.

The result was a lost month, when the world’s richest country — armed with some of the most highly trained scientists and infectious-disease specialists — squandered its best chance of containing the virus’s spread. Instead, Americans were left largely blind to the scale of a looming public-health catastrophe.

The absence of robust screening until it was “far too late” revealed failures across government, said Dr. Thomas Frieden, a former CDC director. Jennifer Nuzzo, an epidemiologist at Johns Hopkins, said the Trump administration had “incredibly limited” views of the pathogen’s potential impact. Dr. Margaret Hamburg, a former commissioner of the U.S. Food and Drug Administration (FDA), said the lapse enabled an “exponential growth of cases.”

And Dr. Anthony S. Fauci, a top government scientist involved in the fight against the virus, told members of Congress that the early inability to test was “a failing” of the administration’s response to a deadly, global pandemic. “Why,” he asked later in a magazine interview, “were we not able to mobilize on a broader scale?”

Across government, they said, three agencies responsible for detecting and combating threats like the coronavirus failed to prepare quickly enough. Even as scientists looked at China and sounded alarms, none of the agencies’ directors conveyed the urgency required to spur a no-holds-barred defence.

Dr. Robert R. Redfield, a former military doctor and prominent AIDS researcher who directs the CDC, trusted his veteran scientists to create the world’s most precise test for the coronavirus and share it with state laboratories. When flaws in the test became apparent in February, he promised a quick fix, though it took weeks to settle on a solution.

The CDC also tightly restricted who could get tested and was slow to conduct “community-based surveillance,” a standard screening practice to detect the virus’s reach.

Had the U.S. been able to track its earliest movements and identify hidden hot spots, local quarantines might have confined the disease.

FDA commissioner Dr. Stephen Hahn enforced regulations that paradoxically made it tougher for hospitals, private clinics and companies to deploy diagnostic tests in an emergency. Other countries that had mobilized businesses were testing tens of thousands daily, compared with fewer than 100 on average in the U.S., frustrating local health officials, lawmakers and desperate Americans.

At the start of that crucial lost month, when his government could have rallied, U.S. President Donald Trump was distracted by impeachment and dismissive of the threat to the public’s health or the country’s economy.

By the end of the month, Trump claimed the virus was about to dissipate in the U.S., saying: “It’s going to disappear. One day — it’s like a miracle — it will disappear.”

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By early March, after federal officials finally announced changes to allow more expansive testing, it was too late to escape serious harm.

Now, the U.S. has more than 100,000 coronavirus cases, the most of any country in the world. Yet even with deaths on the rise, cities shuttered, the economy sputtering and everyday life upended, many Americans who come down with symptoms of COVID-19 still cannot get tested.