NEW YORK — Chipotle shares extended their slide Monday as an E. coli outbreak linked to its restaurants has the company facing its first decline in a key sales figure.

The chain known for touting its quality ingredients had said Friday it expects sales to drop between 8 and 11 percent at established locations for the fourth quarter if current trends continue.

That would mark the first time the figure fell since Chipotle went public in 2006.

Chipotle’s stock closed down 1.7 percent at $551.75 Monday.

In a filing with the Securities and Exchange Commission, Chipotle said sales have been “extremely volatile” since it temporarily closed restaurants in Oregon and Washington in early November as a result of the outbreak. Sales plummeted by as much as 22 percent as additional cases were reported, and were down 16 percent for November.

The Centers for Disease Control and Prevention has said the outbreak has sickened 52 people in nine states. It has not yet identified the ingredient that made people sick.

The first cases were reported at the end of October, with the most recent illness starting on Nov. 13

About 48 million people get sick from a foodborne illness each year, according to the CDC.