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.