Twitter has engaged Sard Verbinnen & Co. for PR counsel as the company manages the resignation of CEO Dick Costolo.

dick costoloCostolo quit on Thursday, effective July 1, paving the way for co-founder Jack Dorsey, CEO of payments service Square, to return as CEO on an interim basis as a search is launched.

Sard Verbinnen chairman and CEO George Sard in New York and managing director Paul Kranhold, based in San Francisco, are advising Twitter through the change.

Costolo, who will remain on Twitter's board, said he was "tremendously proud" of the company and staff for its accomplishments over his six-year tenure. He helped formulate the microblogging service's business plan and led it through its 2013 initial public offering.

But the company has faced heat from Wall Street and investors over sluggish growth and ad revenue. Although its upper management has turned over in the past year, a lengthy blog post by key investor Chris Sacca last week may have been the tipping point for a CEO change, although Costolo had reportedly been eying an exit for at least a year.

Board member Peter Currie will lead a search committee, noting in a statement that the board will find "the right CEO to lead Twitter into its next phase of growth."