The newspaper said that a federal investigator also became sick a day after entering the Tulane National Primate Research Center near New Orleans in January and tested positive for the bacteria, Burkholderia pseudomallei, but it was unclear whether she had been exposed to the bacteria at the center or beforehand. Health officials said there was no evidence of a threat to public health, USA Today reported.

"We're taking this extraordinarily seriously. It's very disturbing to us," the center's director, Andrew Lackner, told the newspaper.

"Right from the beginning we've spent an enormous amount of time trying to figure out how this could have happened."

The monkeys were housed in the veterinary clinic of the center, which is about 40 miles north of New Orleans. Preliminary tests on a fifth rhesus macaque monkey in the clinic also suggested a possible infection with the bacteria, USA Today said, citing an email shared among federal and state investigators.

Two of the four other monkeys that tested positive for the bacteria in recent months had to be euthanized, the paper reported, quoting officials at the center.

Reuters could not independently verify the report and the center could not be immediately reached for comment. The U.S. Centers for Disease Control and Prevention (CDC) also could not be reached.

The bacteria can cause a disease called Melioidosis in both humans and animals, which has a wide range of symptoms that can be confused with other diseases like tuberculosis or pneumonia, according to the CDC.