On April 9th, 1995, Timothy McVeigh blew-up the Murrah Federal Building in Oklahoma City with a concoction of fertilizer and racing fuel. One-hundred-sixty-eight people were killed, including 19 children. McVeigh said he committed the deed as an act of war against the United States of America.
McVeigh's crime was the largest non-military slaughter of Americans by an American (or Americans).
The second most disastrous nonmilitary slaughter of American by Americans occurred at Mountain Meadows in Southern Utah, September 8-14, 1857. In that incident at least 127 men, women, and children were massacred by order of the leadership of the Mormon Church.