Dallas police chief David O. Brown, whose calm leadership made him the face of the Texas city in the tense days after a sniper killed five police officers, announced his retirement. Chief Brown will step down in October after more than six years as the top cop at the country’s seventh-largest police department, and 33 years on the force. “Serving the citizens of Dallas in this noble profession has been both a true honor and a humbling experience,” he said in a statement released by Dallas police Thursday. Chief Brown, 55 years old, didn’t offer details on his plans after he leaves the force, or specific reasons for his departure. But because of the intense and political nature of the job, police chiefs in major cities rarely last as long Chief Brown.