who had overseen the construction of Fort Ellsworth, and Gen. John Newton, who was in charge of the forts south of Four Mile Run, supervised the construction and managed the flow of men and material. It was laid out in September 1861 with a perimeter of , covering , and was surrounded by abatis, and rifle pits. The fort built by the 27th New York Volunteer Infantry Regiment. It had 31 guns, including four 200 pounders, (8-inch (203 mm) Parrott rifles), and 16 mortars. It was named for Brigadier General Nathaniel Lyon, who was killed at the Battle of Wilson's Creek, Missouri, August 10, 1861. Wartime use General Samuel P. Heintzelman was headquartered at Fort Lyon, southwest of Alexandria, when he was in charge of the defense of Washington from 27 October 1862 to 13 October 1863. Robert Knox Sneden served there on his staff, from January 12, 1862 to March 22, 1862, when they embarked for the Peninsula Campaign. On June 9, 1863, Fort Lyon was rocked by an enormous black powder explosion that resulted in the deaths of 25 soldiers and the destruction of eight tons of powder and several thousand rounds of ammunition. The explosion was loud enough to be heard in nearby Alexandria, and one witness noted: ...about two o'clock today..we were startled by a most violent thundering explosion, followed by another, in quick succession, the earth shook and trembled... I was so frightened...a shell burst very near, for a little stream of blue smoke came in one door and passed out the other... I looked up at Fort Lyon, which at that moment went up with a tremendous shock...It...looked...like the pictures of Vesuvus [sic] during an eruption... Everything flew up from the center and seemed to stand still for a moment...then...pieces of steel, stones, and dirt, came rattling, and thundering down... In the wake of the explosion, Fort Lyon, and Alexandria were visited by many military and civilian dignitaries, including Secretary of War Edwin Stanton and President Abraham Lincoln. The garrison was originally the 16th New York Volunteer Infantry Regiment, 26th New York Volunteer Infantry Regiment, 27th New York Volunteer Infantry Regiment, 107th New York Volunteer Infantry, 142nd Ohio Infantry, 15th New York Heavy Artillery Regiment. In October 1864, Company's G, H, K, and M, 1st Wisconsin Heavy Artillery Regiment were assigned to the Fort. They were mustered out on June 26, 1865. Post-war Nothing