Using advanced technology that allows them to rapidly identify the tiny mutations that the virus makes in its virulent path through human hosts, the scientists working in Washington and several other states made two disconcerting discoveries.

The first was that the virus brought in by the man from Wuhan — or perhaps, as new data has suggested, by someone else who arrived carrying a nearly identical strain — had managed to settle into the population undetected.

Then they began to realize how far it had spread. A small outbreak that had established itself somewhere north of Seattle, they realized as they added new cases to their database, was now responsible for all known cases of community transmission they examined in Washington State in the month of February.

And it had jumped.

A genetically similar version of the virus — directly linked to that first case in Washington — was identified across 14 other states, as far away as Connecticut and Maryland. It settled in other parts of the world, in Australia, Mexico, Iceland, Canada, the United Kingdom and Uruguay. It landed in the Pacific, on the Grand Princess cruise ship.

The unique signature of the virus that reached America’s shores in Seattle now accounts for a quarter of all U.S. cases made public by genomic sequencers in the United States.

With no widespread testing available, the high-tech detective work of the researchers in Seattle and their partners elsewhere would open the first clear window into how and where the virus was spreading — and how difficult it would be to contain.

Even as the path of the Washington State version of the virus was coursing eastward, new sparks from other strains were landing in New York, in the Midwest and in the South. And then they all began to intermingle.