By Jacqueline Howard, Maggie Fox and Sandi Sidhu, CNN

(CNN) -- The novel coronavirus that causes Covid-19 probably spread to people through an animal, and probably started spreading among humans no more than a month or two before it was noticed in December of 2019, according a new 120-page report from the World Health Organization.

But the search for the origin of the virus is ongoing.

"As far as WHO is concerned, all hypotheses remain on the table," WHO Director-General Tedros Adhanom Ghebreyesus said in a statement Tuesday. "This report is a very important beginning, but it is not the end. We have not yet found the source of the virus, and we must continue to follow the science and leave no stone unturned as we do."

The report, released Tuesday, says a scenario where it spread via an intermediate animal host, possibly a wild animal captured and then raised on a farm, is "very likely."

But the investigation has not found what other animal was infected by a bat -- considered the most likely original source of the virus -- and then may have transmitted it to a human. "The possible intermediate host of SARS-CoV-2 remains elusive," the report reads.

Next likely is direct transmission from one of the animals known to carry a similar coronavirus, such as a bat or a pangolin.

The report says introduction through cold food products is considered a "possible pathway" and introduction through a laboratory incident was considered to be "an extremely unlikely pathway."

Without providing evidence, former US Centers for Disease Control and Prevention Director Dr. Robert Redfield told CNN's Dr. Sanjay Gupta that his personal opinion was the virus was released from a lab, not necessarily intentionally.

The report was written by a joint international team made up of 17 Chinese experts plus 17 experts from other countries, WHO, the Global Outbreak Alert and Response Network (GOARN) and the World Organization for Animal Health (OIE). The United Nations Food and Agriculture Organization of the United Nations (FAO) participated as an observer.

Tedros said in prepared remarks that to understand the earliest cases, "scientists would benefit from full access to data including biological samples from at least September 2019.

"In my discussions with the team, they expressed the difficulties they encountered in accessing raw data. I expect future collaborative studies to include more timely and comprehensive data sharing," Tedros added.