As the world does its best to cope with the coronavirus epidemic, researchers are trying to get a grasp on when and where the virus actually started. So far, 3,5 million positive cases and 252,000 deaths have been recorded across the globe, and a further 1,7 million people who have recovered.

New evidence suggests that the coronavirus might have made its way to France a month earlier than previously thought. Doctors in Paris said a patient admitted with pneumonia in December might have been suffering from COVID-19, suggesting the virus was circulating in France at that time.

French researchers led by Yves Cohen, head of resuscitation at the Avicenne and Jean Verdier hospitals, retested samples from 24 patients treated in December and January who had tested negative for flu before COVID-19 developed into a pandemic. The results showed that one patient, a 42-year-old man born in Algeria who had lived in France for many years and worked as a fishmonger, was infected with COVID-19 “one month before the first reported cases in our country”, the researchers said.

The World Health Organization said not to be surprised by the results. “It’s also possible there are more early cases to be found,” WHO spokesman Christian Lindmeier told a U.N. briefing in Geneva. He encouraged other countries to check records for cases in late 2019, saying this would give the world a “new and clearer picture.”

Europe did not start reporting cases of coronavirus until January. In Italy, the European country hit hardest by the virus, the first two cases were reported on January 31, in two Chinese tourists in Rome. The first known community transmission was recorded at the end of February in Codogno, northern Italy.

The patient in France had not been to China, and one of his children had also been sick. His last trip was in Algeria during August 2019. This suggests that the disease was already spreading among the French population at the end of December, the researchers argued.

“Identifying the first infected patient is of great epidemiological interest as it changes dramatically our knowledge regarding SARS-COV-2 and its spreading in the country,” they added.

Nevertheless, independent experts that weren’t involved in the study said more investigation is needed to be sure about an earlier presence of the virus. Jonathan Ball, a professor of molecular virology at Britain’s University of Nottingham, told Reuters the evidence so far is inconclusive.

Stephen Griffin, an expert at the University of Leeds’ Institute of Medical Research, told French television that this was a “potentially important finding” but asked to be cautious to interpret it. He argued it’s still too early to tell if the patient was France’s “patient zero.”

The study from the French researchers was published in the International Journal of Antimicrobial Agents.