(Photo by Joe Raedle/Getty Images)

Chipotle says that a sick employee likely caused the outbreak of norovirus at the chain’s Sterling, Va. location earlier this month.

“We believe someone was working while sick,” said CEO Steve Ells on a call with Wall Street analysts on Tuesday evening.

Two people who ate at the Chipotle on Tripleseven Road between July 13 and 16 have tested positive for the same strain of norovirus, and the Loudoun County Health Department has identified more than 135 people who reported becoming ill with symptoms like diarrhea, fevers, nausea, and vomiting.

The Sterling location closed for two days “to conduct a complete sanitization,” according to an emailed statement from Jim Marsden, Chipotle’s director of food and safety, last week.

Now, the company has been served with a federal grand jury subpoena for information regarding the outbreak in Sterling as part of a wider investigation into food-related illnesses at Chipotle, according to Business Insider.

Chipotle’s CEO maintained that the company has “excellent” health protocols, including a paid sick-leave policy, and is now focusing on ensuring that employees are trained in that policy.

CNBC reports that Chipotle managers aren’t always abiding by them, though.

An employee from another Virginia location told CNBC that, “Two weeks ago [I] came to work, felt like I was going to puke, just felt awful. One of my managers told me if I don’t find someone to cover my shift, I’m going to have to stay. Mind you, my position was on line working with food.”

But after a rough week in the news, Chipotle is now pledging to ensure its more than 2,200 branches stick with the protocols. “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.”

An analysis of D.C.’s implementation of paid sick leave saw a drop in rates of flu and other contagious diseases by an additional 5 percent after the law was extended to include waitstaff and other tipped workers.