Investigators from the World Health Organization (WHO) looking into the origins of coronavirus in China have discovered signs the outbreak was much wider in Wuhan in December 2019 than previously thought, and are urgently seeking access to hundreds of thousands of blood samples from the city that China has not so far let them examine.

The lead investigator for the WHO mission, Peter Ben Embarek, told CNN in a wide-ranging interview that the mission had found several signs of the more wide-ranging 2019 spread, including establishing for the first time there were over a dozen strains of the virus in Wuhan already in December. The team also had a chance to speak to the first patient Chinese officials said had been infected, an office worker in his 40s, with no travel history of note, reported infected on December 8.

Changes in a virus’s genetic makeup are common and normally harmless, occurring over time as the disease moves between and reproduces among people or animals. Embarek declined to draw conclusions about what the 13 strains could have meant for the disease’s history before December.

But the discovery of so many different possible variants of the virus could suggest it had been circulating for longer than just that month, as some virologists have previously suggested. This genetic material is likely the first physical evidence to emerge internationally to bolster such a theory.