A genetically similar version of the virus — directly linked to that first case in Washington — was identified across 14 other states and around the world.

Related: We compared virus-related death rates across the U.S., in which the Northeast accounts for about two-thirds of fatalities.

Another angle: Two new studies using antibody tests to assess how many people have been infected have turned up higher numbers than some experts had expected.

China helped sow panic, U.S. officials say

Alarming messages that popped up last month on cellphones and social media feeds of millions of Americans warned that the Trump administration was about to lock down the entire country.

U.S. intelligence agencies have since determined that the warnings were amplified by Chinese operatives, prompting officials to look at new ways in which countries are encouraging the spread of disinformation during the pandemic.

American officials said the operatives had adopted some of the techniques mastered by Russia-backed trolls, such as creating fake social media accounts to push messages to receptive Americans, who in turn unwittingly help spread them.