As the number of Union forces increased, requiring better organization, Maj. Gen. George B. McClellan was appointed as commander of the Army of the Potomac defending Washington. He brought with him as his chief of intelligence Allan Pinkerton, who had gained some renown running a Chicago detective agency. Pinkerton, often using the alias Major Allen or E. J. Allen, had responsibilities for collecting intelligence on the enemy and for counterintelligence activities against enemy agents. He gained most of his intelligence by conducting an extensive and well-organized debriefing program of people crossing over from Confederate lines. These informants included merchants with business ties on both sides, deserters from the Confederate Army, prisoners of war, civilians traveling to escape the fighting or for other personal business, and former slaves. While each group provided valuable information, Pinkerton soon discovered that the former slaves were the most willing to cooperate and often had the best knowledge of Confederate fortifications, camps, and supply points.