Flickr/b-duss Chipotle is doubling down on free-food offers to lure customers back to its restaurants.

The company is sending out 21 million direct-mail coupons for free food in the coming weeks, Chipotle executives said Wednesday at a conference in New York. The coupons will expire on May 15.

The chain said it had success with its most recent free-burrito promotion. About 5.3 million people downloaded the mobile coupon in February, with 2.5 million redeeming the offer.

"We fully expected that [offer] to go viral," Mark Crumpacker, Chipotle's chief creative officer, said. "That was our first test to see how much people really wanted to come back to Chipotle."

In addition to the direct-mail coupons, Chipotle will also most likely send out more mobile promotions for free burritos.

"If we don't see the number of customers that we would like to see returning to restaurants, we may do more like the [February mobile offer] in regional places around the country," Crumpacker said.

Restaurant traffic has been lagging particularly on the East and West coasts, so those regions will most likely be targeted for mobile coupons.

In the summer, the chain may shift to buy-on, get-one offers, executives said.

Chipotle is offering free food to combat falling restaurant traffic and sales after two E. coli outbreaks linked to its restaurants sickened more than 50 people in 14 states.

"Free burritos — turns out it works," Chipotle CFO Jack Hartung said. "It brings people into the restaurants."

Chipotle's same-store sales, or sales at restaurants open at least a year, fell 26.1% in February, following a 36.4% drop in January, the company said Tuesday.

In the first half of March, same-store sales were down about 22%. After it was reported that a store in Boston closed because of sick employees, the declines dropped to 27%.