DENVER — Chipotle says it believes an employee was working while sick at a Virginia location where dozens of customers reported becoming ill earlier this month with what health officials think was norovirus.

After the company reported improved sales for the second quarter Tuesday, CEO Steve Ells said that a company investigation into the illnesses found that its leadership at the store didn’t adhere to its protocols.

“We believe someone was working while sick,” Ells said, without giving specifics. Health officials had said Monday the specific cause of norovirus hasn’t been identified.

A Chipotle representative noted that employees are not supposed to work when ill, and the company offers paid sick days for hourly employees. Ells said the company is adding training to what he called “excellent” protocols to prevent further norovirus cases.

Chipotle’s image has been vulnerable as it tries to reassure customers its food is safe since an E. coli outbreak in fall 2015, which was followed by a norovirus outbreak at a single restaurant that same year. Norovirus is a leading cause of illnesses from contaminated food, and infected employees are a frequent source.

The Denver-based chain has made progress in winning back customers, and said sales rose 8 percent during the second quarter, following a 24 percent decline in the year-ago period.

But the most recent figure doesn’t reflect any potential fallout from the reported illnesses in Sterling, Virginia.