The “window of opportunity” for finding the cause of the COVID-19 outbreak is “closing fast,” said a team of international scientists appointed by the World Health Organization to conduct the search.

In a Wednesday article in the scientific journal Nature, the group of experts said the search has been stalled and warned that continued delay could make “studies biologically impossible.”

“SARS-CoV-2 antibodies wane, so collecting further samples and testing people who might have been exposed before December 2019 will yield diminishing returns,” they wrote in the article.

The WHO’s group of scientists were dispatched to China earlier this year to investigate the first human COVID-19 cases detected in Wuhan in December 2019, AP News reported.

In their initial analysis published in March, the team concluded that the virus likely passed from animals to humans. They determined a potential laboratory leak as “extremely unlikely.”

However, this analysis was simply a first step in a long, collaborative process that has come to a standstill. They noted that Chinese officials are still reluctant to share some raw data, citing concerns over patient confidentiality.

China said at a Wednesday press briefing that officials should “concentrate on other possible avenues that may help trace the origin” of COVID-19 and suggested studies should be pursued in other countries.

Yet, the world wants answers and WHO is not the only group conducting investigations on the origin of the coronavirus outbreak.

President Joe Biden ordered a U.S. intelligence review of the pandemic's origin. He received a classified report on Tuesday stating that the search was inconclusive, including whether the pathogen jumped from an animal to a human as part of a natural process, or escaped from a lab in central China, The Washington Post reported.

However, an official told The WP the intelligence community is “not necessarily best equipped to solve this problem,” which is fundamentally an issue of science. Although spy services are “positioned to collect on a range of foreign actors” they are not necessarily poised to dive into global health data sets.

With this, answers on how the COVID-19 outbreak started remain a global mystery and the clock is ticking as the search timeline is quickly closing.

Scientists called on “the scientific community and country leaders to join forces to expedite” their search.