The study published Monday identified 106 infections from 7,389 blood samples collected from donors in nine U.S. states between Dec. 13 and Jan. 17. The samples, collected by the American Red Cross, were sent to the U.S. Centers for Disease Control and Prevention for testing to detect if there were antibodies against the virus.

“The findings of this report suggest that SARS-CoV-2 infections may have been present in the U.S. in December 2019, earlier than previously recognized,” the paper said.

Reports of a mysterious pneumonia spreading in Wuhan, China, first emerged in late December 2019. After multiplying rapidly throughout the city in the following weeks, the disease spread across the globe, with the first U.S. case emerging on Jan. 19.

The revelations in the paper by researchers from the CDC reinforce the growing understanding that the coronavirus was silently circulating worldwide earlier than known, and could re-ignite debate over the origins of the pandemic.

It’s not the first evidence showing the virus could have existed or infected people outside China before 2020. A patient in France was found to have contracted the virus after being hospitalized with flu-like symptoms at the end of December, contradicting official statistics showing Covid-19 reached the country from people returning from Wuhan at the end of January.

The CDC study indicated there were isolated infections in the western part of the U.S. in mid-December. Antibodies were also found in early January in other states before the virus was known to have been introduced to those places.

The scientists indicated it’s unlikely that the antibodies developed to curb other coronaviruses, as 84 samples were found to have neutralizing activity specific to SARS-CoV-2.

They also noted it wasn’t possible to determine the magnitude of infections on a state or national level based on the samples, or whether the cases were locally transmitted or travel-related.