Company officials say Ms. Saenz was fired on April 29, 2008, after a patient spotted her injecting an unusual fluid into the intravenous tube of another person undergoing dialysis.

Image Kimberly Saenz

A month later, she was arrested on assault charges in connection with patients who had become ill after receiving infusions of bleach. At the time, the police said at least two patients had seen her draw bleach into syringes and inject it into patients. Traces of bleach were also found in syringes and dialysis lines, they said.

On Tuesday, the Angelina County district attorney, Clyde Herrington, convinced a grand jury that the evidence against Ms. Saenz was strong enough to charge her with murder in five deaths. She was arrested Thursday night and was being held in the Angelina County jail without bond. Her lawyer, John Henry Tatum, did not respond to messages left at his office in Lufkin, a town of 40,000 people about 115 miles north of Houston.

DaVita, which operates 1,400 dialysis clinics across the country, maintains there was nothing in Ms. Saenz’s employment history to suggest she was a danger to patients. The company has also said managers at the clinic could not have prevented the attacks.

“This lone individual’s alleged intentional and deceptive acts have caused incredible grief for the victims’ family members,” a spokesman for the company, Richard A. Grenell, said in a statement.