A U.S. intelligence report identified three researchers at a Wuhan lab who sought treatment at a hospital after falling ill in November 2019, a source familiar with the matter tells NBC News.

The new details, first reported by The Wall Street Journal, add to the body of circumstantial evidence that supports a hypothesis that the Covid-19 virus may have spread to humans after it escaped from a Chinese research lab in Wuhan. But the evidence is far from conclusive.

This reporting adds a new level of detail to the information put out in a Department of State fact sheet last year, which said the U.S. had confirmed that researchers at the Wuhan Institute of Virology had fallen ill.

“The U.S. government has reason to believe that several researchers inside the WIV became sick in autumn 2019, before the first identified case of the outbreak, with symptoms consistent with both Covid-19 and common seasonal illnesses. This raises questions about the credibility of WIV senior researcher Shi Zhengli’s public claim that there was “zero infection” among the WIV"s staff and students of SARS-CoV-2 or SARS-related viruses”, - the fact sheet said.

In February, NBC News reported that the U.S. intelligence agencies had not ruled out the possibility of a lab accident, noting that a growing number of scientists are urging an investigation into that possibility. The other theory is that the virus was transmitted to humans through an animal host. After a year and half of looking, no such host has been identified.

U.S. officials say that unless China allows more access to records and people, they may never get to the bottom of it.