Inmates at a facility in Castaic, north of Los Angeles, were filmed sharing a disposable cup and sniffing a used face mask while crowded together.

"Somehow, there was some mistaken belief among the inmate population that if they tested positive, that there was a way to force our hand and somehow release more inmates out of our jail environment -— and that's not going to happen," said county sheriff Alex Villanueva.

Some 21 prisoners tested positive in the prison section where the video was shot "as a result of the behavior," he added.

The activity came to light as prison officials investigated a broader spike in COVID-19 cases behind bars, with nearly 40 percent of those incarcerated in Los Angeles County now in quarantine.

Los Angeles County currently has 11,700 inmates, after more than 5,000 prisoners were released in a preemptive measure to avoid a catastrophic coronavirus outbreak.

Still, 357 prisoners have tested positive in the county, which has the United States' largest prison population.

The footage was "just a sampling of the many other videos being reviewed and used in the ongoing investigation," said John Satterfield, a lieutenant in the sheriff's department.

Another video showed prisoners sipping hot water in a bid to raise their temperatures before nurses conducted a medical check.

The men were "trying to falsely elevate their temperature readings" to emulate one of the disease's symptoms, Villanueva said.

None of the prisoners has admitted to seeking to infect themselves.

Prison reform advocates denied the claim, and accused Villanueva of seeking to "demonize incarcerated people."

The sheriff's "shameful" claims "sensationalize the acute fear of the incarcerated population," said campaigner Patrisse Cullors in a statement calling for the release of more inmates.

It was not clear how the outbreak at the North County Correctional Facility in Castaic began.