A month-long World Health Organization investigation into the origin of COVID-19 has painted "a classic picture of [...] an outbreak," team members said on Tuesday evening as they presented their initial findings into the source of the global pandemic.

The initial source of COVID-19 was probably an animal, which spread the virus to humans via zoonotic transmission in food markets in the city.

"The search for a possible root of the infection through different animal species and a specific reservoir is still a work in progress," said WHO team leader and Danish scientist Peter Ben Embarek.

A formal report could take a week or more to release, according to reports.

The investigation tracked goods being sold by Huanan Market wildlife vendors to their places of origin throughout China and even abroad, but still does not know the exact source for the virus.

"Beyond the Huanan Seafood Market there was no obvious cluster of exposure," said Dr. Liang Wannian, head of China's pandemic response team within the China National Health Commission (NHC).

Bats, pangolins and feline species are all known natural reservoirs of coronaviruses, according to Liang, and more study will be required to narrow the field.

However, as Wuhan is an urban center, the virus most likely did not make a direct jump from an animal carrier to city dweller. There had to have been an intermediary, making an earlier outbreak in a rural region before fall 2019 likely.

"Some samples were detected even earlier. This indicates the possibility of misreported circulation in other regions," said Liang, who noted that Wuhan's mortality rates showed no signs of a fast-spreading disease before October.

Concluding the WHO's month-long investigation, team leader Embarek shone a spotlight on the people of Wuhan for enduring so much.

"The international team would like to recognize the effect of the pandemic on the city of Wuhan," he said and singled out the region's medical staff and first responders for their work.

When cases first appeared in late 2019 and early 2020, Chinese authorities denied any suggestion of an epidemic in Wuhan, and prevented doctors from sharing information about the virus, jailing those who disobeyed.

The country blocked earlier attempts to allow an international investigative team into the country to examine the source of the pandemic that has killed more than 2.3 million people worldwide since December 2019. Beijing however has bristled at criticism of its handling of the outbreak.

"China has always shown firm support of the WHO in terms of its COVID-19 response," said Mi Feng, a spokesman for China's National Health Commission, Tuesday.