Health officials said that norovirus is believed to be what caused dozens of people to report becoming ill after eating at a Chipotle in suburban Washington, D.C., earlier this month.

The Loudoun County Health Department in Virginia said Monday that a second person has tested positive for norovirus, and it was the same strain as the first person whose test was positive. The department says it identified more than 135 people who reported becoming ill after visiting the Chipotle in Sterling, Virginia, between July 13 and 16.

The health department said, however, that it has not identified the specific source of the norovirus. It says it is not aware of any customers becoming ill since the store in Sterling reopened last Wednesday, after undergoing what the restaurant chain called "complete sanitizations."

Chipotle Mexican Grill, which has struggled to recover from past food scares, has noted its procedures for dealing with such issues quickly, and said its food is safe to eat. Its stock nevertheless has fallen since the illnesses were reported, underscoring how vulnerable it remains. The shares were down again Monday.

The Denver-based chain has been trying to win back customers since an E. coli outbreak in fall 2015, which was followed by a norovirus outbreak at a single restaurant later that same year.

Norovirus is a leading cause of illnesses from contaminated food, and infected employees are a frequent source of outbreaks, according to the Centers for Disease Control and Prevention. The agency says one out of six Americans get sick each year by consuming contaminated food or drinks more broadly.

David Goodfriend, director of the Loudoun County Health Department, said in an email that some of those sickened were Chipotle employees.