A top NIH official admitted in a Wednesday letter that U.S. taxpayers funded gain-of-function research on bat coronaviruses in Wuhan and revealed that EcoHealth Alliance, the U.S. non-profit that funneled NIH money to the Wuhan Institute of Virology, was not transparent about the work it was doing.

In the letter to Representative James Comer (R., Ky.), Lawrence A. Tabak of the NIH cites a “limited experiment” that was conducted to test if “spike proteins from naturally occurring bat coronaviruses circulating in China were capable of binding to the human ACE2 receptor in a mouse model.” The laboratory mice infected with the modified bat virus “became sicker” than those infected with the unmodified bat virus.

The revelation vindicates Republican senator Rand Paul, who got into heated exchanges with National Institute of Allergy and Infectious Disease director Anthony Fauci during his May and July testimonials before Congress over the gain-of-function question. At the second hearing, Paul accused Fauci of misleading Congress by denying that the U.S. had funded gain-of-function projects at the Wuhan Institute of Virology.

Gain-of-function research involves extracting viruses from animals and artificially engineering them in a laboratory to make them more transmissible or deadly to humans.