Chipotle Mexican Grill has hired Carrot Creative as its first social media agency of record after a competitive pitch involving six agencies.

The appointment comes as Chipotle tries to move past food safety scares, which have kept many diners away. On Wednesday, the chain ran a full-page apology letter in about 60 newspapers, and issued a link to the letter from its @ChipotleTweets account.

Carrot will handle social content, analytics, strategy and consulting, monitoring and some paid content across channels such as Twitter, Facebook, Snapchat and Instagram as of January, said Jackson Jeyanayagam, Chipotle's director-digital marketing. The firm could also potentially do some social influencer work for the brand, which is known for focusing on online and in-store marketing rather than TV.

When asked if Carrot will do any work around Chipotle's recent E. coli outbreak, he said the shop's duties will "not be reactive."

GSD&M, Chipotle's lead media agency since March 2014, will continue handle digital media buying.

Mr. Jeyanayagam, who joined the restaurant chain in May, said he wanted to bring on a social agency to support Chipotle's lean internal team and help the brand "do some really cool things next year."

"What you hear from us on social will sound the same as in a restaurant, a game or piece of content, but our audience lives and breathes social, and they're there constantly trying new things, so we have to be there too and continue to be relevant with them," he said.

He declined to disclose budget information.

Chipotle said earlier this month that its E. coli outbreak has taken a toll on its sales, causing steep sales dips at existing locations and leading the company to rescind its 2016 sales forecast.