KLAMATH FALLS, Ore. (AP) — A Klamath Falls man has pleaded guilty to sending cards to former coworkers containing a white powdered pesticide threatened to be anthrax, prosecutors said.

Kelly Michael Burns, 71, pleaded guilty Monday to eight counts of mailing threatening communications, according to the U.S. Attorney’s Office for the District of Oregon.

In December 2019, Burns mailed four Christmas cards, postmarked in Medford, to former coworkers containing a white powder pesticide later identified as carbaryl, a known pesticide toxic to humans, court documents said. The cards were addressed to the victims’ workplace and contained violent threats.

Three people including a pregnant woman at Burns’ former workplace were exposed to the carbaryl, had to go through a decontamination process, and were given a high-dose antibiotic, authorities said.

The company was evacuated and closed for decontamination.

In February 2020, Burns again mailed cards containing carbaryl to the same four victim recipients. This second wave of cards also contained violent statements with added threats directed at the victims’ families.

FBI handwriting analysts matched the handwriting on the cards to his handwriting obtained from a job application and tax documents.

In March 2020, FBI agents in a search of his residence found handwritten notes including one in which Burns threatened to kill a former coworker. Agents also recovered several books including, “The Poisoner’s Handbook.”

Sentencing is set for May.