LOS ANGELES — Chipotle closed all of its U.S. restaurants for part of the day Monday so its employees could attend a company-wide meeting to discuss food safety changes and staff could ask questions.

All restaurants of the Denver-based burrito chain were to reopen at 3 p.m. in their respective time zones. As of December, Chipotle Mexican Grill Inc. has 1,971 locations across the nation.

Chipotle’s sales have slumped since two E. coli outbreaks were tied to its restaurants. In November, Chipotle temporarily closed 43 restaurants in Washington state and Oregon after 22 cases were linked to its eateries. Those locations have since reopened.

In December, there was a second, smaller outbreak of E. coli tied to Chipotle restaurants. A total of five people were affected in Kansas, North Dakota and Oklahoma.

Last week, the federal Centers for Disease Control and Prevention said the two outbreaks tied to the chain appeared to be over. The CDC said it was closing its investigation, though it was unable to determine a food or ingredient responsible for the contamination.

In December, the company took another hit when 141 Boston College students were reported to have contracted norovirus after eating at a nearby Chipotle restaurant.

Chipotle was also subpoenaed by a federal grand jury for documents relating to a norovirus outbreak in August at a Los Angeles-area restaurant. Chipotle has said it will fully cooperate.