WASHINGTON — In the final days before the United States faced a full-blown epidemic, President Trump made a last-ditch attempt to prevent people infected with the coronavirus from reaching the country.

“To keep new cases from entering our shores,”

Trump said in an Oval Office address on March 11, “we will be suspending all travel from Europe to the United States for the next 30 days.”

Across the Atlantic, Jack Siebert, an American college student spending a semester in Spain, was battling raging headaches, shortness of breath and fevers that touched 104 degrees. Concerned about his condition for travel but alarmed by the president’s announcement, his parents scrambled to book a flight home for their son – an impulse shared by thousands of Americans who rushed to get flights out of Europe.

Siebert arrived at O’Hare International Airport in Chicago three days later as the new U.S. restrictions – including mandatory medical screenings – went into effect. He encountered crowds of people packed in tight corridors, stood in lines in which he snaked past other travelers for nearly five hours and tried to direct any cough or sneeze into his sleeve.

When he finally reached the coronavirus checkpoint near baggage pickup, Siebert reported his prior symptoms and described his exposure in Spain. But the screeners waived him through with a cursory temperature check. He was given instructions to self-isolate that struck him as absurd given the conditions he had just encountered at the airport.

“I can guarantee you that people were infected” in that trans-Atlantic gantlet, said Siebert, who tested positive for the virus two days later in Chicago. “It was people passing through a pinhole.”

The sequence was repeated at airports across the country that weekend. Harrowing scenes of interminable lines and unmasked faces crammed in confined spaces spread across social media.

The images showed how a policy intended to block the pathogen’s entry into the United States instead delivered one final viral infusion.

As those exposed travelers fanned out into U.S. cities and suburbs, they became part of an influx from Europe that went unchecked for weeks and helped to seal the country’s coronavirus fate.

Epidemiologists contend the U.S. outbreak was driven overwhelmingly by viral strains from Europe rather than China. More than 1.8 million travelers entered the United States from Europe in February alone as that continent became the center of the pandemic.

Infections reached critical mass in New York and other cities well before the White House took action, according to studies mapping the virus’s spread. The crush of travelers triggered by Trump’s announcement only added to that viral load.

“We closed the front door with the China travel ban,” New York Democratic Gov.

Andrew Cuomo said last month as officials began to grasp the magnitude of the failure. In waiting to cut off travel from Europe, he said, “we left the backdoor wide open.”