An intelligence bulletin from the FBI late last year warned of a growing "biosecurity" threat within the United States after Chinese nationals were caught attempting to sneak potentially dangerous viruses into the country by plane.

The “tactical intelligence report” from the FBI’s Weapons of Mass Destruction Directorate assessed in November that “foreign scientific researchers who transport undeclared and undocumented biological materials into the U.S. in personal carry-on and/or checked luggage almost certainly present U.S. biosecurity and biosafety risks,” according to the unclassified document obtained by Yahoo News.

Even when these samples were declared, investigators warned, "It is impossible to determine, without testing, the validity of the contents of the samples and if they pose a risk to U.S. human, animal, or plant populations.”

The bureau’s Chemical and Biological Intelligence Unit pointed to at least three separate instances in 2018 and 2019 where Chinese nationals tried to bring undeclared samples of bacteria and viruses, some of them potentially highly dangerous, into the U.S. All of the failed attempts were stopped by Customs and Border Protection at Detroit Metropolitan Airport.

One of the incidents took place a couple months before the coronavirus outbreak first appeared in Wuhan, China. The U.S. intelligence community believes the Chinese lied about the severity of the COVID-19 outbreak for months and to this day is covering up the real number of cases and deaths in China.

Last year’s FBI report said in September that a Chinese national was stopped after he “initially made no positive declarations, but was later found to have eight vials of clear liquid in their checked luggage.” The bureau noted that “the vials had no supporting documentation.” The Chinese national claimed that it was “DNA … derived from a low-pathogenicity strain of H9N2,” which was a flu-type virus that killed a small number in Asia. But some vials had “WSN” handwritten on top, an acronym for the H1N1 influenza virus commonly known as swine flu, which killed over 12,000 in the U.S. and more globally. The bureau said that “the materials were confiscated" and that the unnamed person was allowed to travel to Texas "to work with a researcher associated with” an unnamed “U.S. research institution.”

The bureau advisory also recounted how, in November 2018, another Chinese national was found with three vials labeled “antibodies” in his luggage. The person identified himself as a “biologist” but “had not declared the materials” and “did not have appropriate documentation for the items.” The Chinese national said that “the items came from a researcher in China who asked him to deliver them to another colleague," again at an unnamed “U.S. research institution.” The writing on the vials combined with their destination led U.S. officials to believe the vials might contain “viable” specimens of Middle East respiratory syndrome and severe acute respiratory syndrome viruses. MERS has killed just under one thousand people worldwide since 2012, and SARS killed just under 800 globally between 2002 and 2004.

FBI investigators further noted that, in May 2018, a Chinese national was stopped. This person claimed to be “a breast cancer researcher in Texas” who “was not traveling with any biological products.” But, upon further inspection, the person admitted to be “possibly traveling with plasmids,” a type of extrachromosomal DNA. He was found to have one “centrifuge tube” in his checked bag, saying it was “non-infectious E. Coli bacteria-derived plasmids.” The bureau said the Chinese national was “unable to provide any accompanying documentation or permits,” so the U.S. officials put the centrifuge on an “agricultural hold” and let him go.

The U.S. government's concerns about Chinese research in the U.S. have grown recently.

In January, the Justice Department announced Charles Lieber, the chairman of Harvard’s chemistry department, was charged with one count of “making a materially false, fictitious, and fraudulent statement” about his connections to China’s Thousand Talents Program, which the FBI has deemed a Chinese form of “nontraditional espionage.”

Lieber, a specialist in nanoscience, received more than $15 million in grants from the National Institutes of Health and the Pentagon, requiring him to disclose foreign financial conflicts of interest. But Lieber became a “strategic scientist” at China’s Wuhan University of Technology in 2011 and was a participant in the Thousand Talents Plan from roughly 2012 to 2017. The foreign university paid Lieber $50,000 per month and living expenses of approximately $158,000 and awarded him more than $1.5 million to establish a research lab at the Chinese school. Lieber was obligated to work for the foreign school “not less than nine months a year” by “declaring international cooperation projects, cultivating young teachers and Ph.D. students, organizing international conference[s], applying for patents, and publishing articles in the name of” the Wuhan University of Technology.

Lieber allegedly falsely told investigators in April 2018 that he was never asked to participate in the Thousand Talents Program and “wasn’t sure” how China categorized him. The Justice Department said Lieber also caused Harvard to tell the NIH that Lieber “had no formal association with” the Chinese university after 2012 and that Lieber “is not and has never been a participant in” China’s Thousand Talents Plan.

The Justice Department simultaneously announced charges against two Chinese nationals.

Zaosong Zheng was a Chinese researcher arrested in December and accused of trying to smuggle vials of cancer cells out of the U.S. The FBI claimed that Zheng stole the vials from a laboratory at Beth Israel Deaconess Medical Center. He was charged with making false statements to investigators.

Yanqing Ye studied at Boston University and was accused of lying to authorities about her status as a lieutenant in the People’s Liberation Army. Ye, who was charged with visa fraud, making false statements, and acting as an agent of a foreign government, wasn’t arrested because she was in China.

The Education Department announced in February that it would be scrutinizing both Harvard University and Yale University over their funding, including money from China and any ties to Chinese telecommunications giant Huawei and the Thousand Talents Program.