Shares of Snyder's-Lance Inc., the snack-food company formed in 2010 by the merger of the once-Hanover, Pa.-based Snyder's pretzel empire and the maker of Lance crackers, plunged almost 20 percent Monday morning after the company announced that Carl Lee had retired unexpectedly as chief executive officer.

The company, now based in Charlotte, N.C., also said it has faced "difficult challenges" that have weighed on first-quarter earnings. At the close of trading on the Nasdaq, the stock closed down 15.43 percent, or 6.16, to $33.76.

Lee, 57, had been with the company for almost 12 years. Brian Driscoll, former CEO of Diamond Foods and a member of the Snyder's-Lance board, has stepped in as interim CEO.

The company said in a statement that it will launch a national search for a permanent replacement for Lee, saying Driscoll is a "strong candidate" for the position.

It did not say whether the company's recent financial performance was the reason for Lee's early retirement.

Alex Pease, chief financial officer, said Snyder's-Lance has faced "difficult challenges" during the first quarter that have hurt earnings. Under Driscoll's leadership, Pease said, the company is moving aggressively to improve earnings.

For the first quarter, Snyder's-Lance expects earnings to be between 11 and 12 cents per diluted share. The company expects net sales in the range of $530 million to $532 million, a rise of about 18 percent to 19 percent from continuing operations in first quarter 2016.

"Increased investments in promotional and marketing spending combined with gross margin pressure had an adverse effect on our performance and more than offset the benefits of synergy delivery related to the Diamond Foods transaction," Pease said.

Driscoll was CEO of Diamond Foods until Snyder's-Lance bought the company in early 2016. Snyder's-Lance said late last year that it was selling Blue Diamond's nut business to a private equity firm, Blue Road Capital.

Lee was CEO of Snyder's of Hanover from 2005 until December 2010, when the Pennsylvania company merged with Charlotte-based Lance. Lee became the merged company's chief operating officer, and assumed the role of CEO in May 2013.

The company, which also produces Stella D'Oro and Archway cookies and Cape Cod potato chips, has been expanding its selection of "better-for-you" offerings with acquisitions such as Diamond.