July 26 (UPI) -- A sick employee spread a viral illness that sickened at least 135 customers at a Chipotle in Virginia, the latest outbreak affecting the restaurant chain.
Chipotle closed down the Sterling, Va., restaurant for two days starting July 17 after customers became sickened. Chipotle conducted an investigation to determine the cause of the highly contagious illness.
CEO Steve Ells said the company's sick policy was violated, CNN reported.
"We believe someone was working while sick," Ells said Tuesday in a call with Wall Street analysts.
Ells said the company's employees will undergo "relentless training" to reinforce that they cannot work while contagious, meeting current protocols he called "excellent."
"We took swift action and made it clear to the entire company that we have a zero-tolerance policy for not following these protocols," Ells said. "Compliance with our procedures is non-negotiable and a condition of their employment."
The company revealed Wednesday that the Food and Drug Administration subpoenaed Chipotle's records of the latest outbreak as part of its criminal investigation, which looks into outbreaks nationwide the past two years.
The company, in a filing with the Securities and Exchange Commission, confirmed it would "fully cooperate in the investigation, CNBC reported.
"It's unusual for [the FDA's office of investigations] to be focused on a particular restaurant," food illness expert and attorney Bill Marler told the New York Post. "I can only assume that they are seeing a similar problem that looks like the [norovirus outbreaks in] Simi Valley and Boston."
Chipotle has had multiple outbreaks of E. coli and norovirus in about a dozen states since 2015, prompting temporary closings of its restaurants and an overhaul of health decorum.
Last week, several mice entered what the company called a "structural gap" in a Dallas Chipotle.