Extract from Maj. Gen. George B. McClellan's report covering operations of the Army of the Potomac from July 27, 1861, to November 9, 1862.
All spies, "contrabands," deserters, refugees and many prisoners of war coming into our lines from the front were carefully examined, first by the outpost and division commanders, and then by my chief of staff and the provost-marshal-general. Their statements taken in writing, and in many cases under oath, from day to day for a long period previous to the evacuation of Manassas comprised a mass of evidence which by careful digests and collations enabled me to estimate with considerable accuracy the strength of the enemy before us
I am, sir, very respectfully, your obedient servant,
Major-General, U.S. Army.