Floyd Lee Corkins II, 28, of Herndon said that when he opened fire Aug. 15 in the lobby of the council's offices, he "intended to kill as many people as possible," court documents showed.
Corkins pleaded guilty in U.S. District Court to a federal charge of transporting a weapon across state lines and to District of Columbia charges of committing an act of terrorism while armed and of assault with intent to kill, court records showed.
He faces up to 30 years in prison on each of the two D.C. charges and up to 10 years on the federal charge, The Washington Post reported. Sentencing is scheduled for April 29.
Corkins pulled a gun from his backpack when Leonardo R. Johnson, the building manager and security guard, asked to see his identification. Corkins shot the unarmed guard once in the arm before Johnson wrestled him to the ground.
After Corkins' arrest, police discovered in his bag a hand-written note with the names and addresses of the Family Research Council and three other conservative organizations. The council opposes gay marriage. The bag also contained two loaded magazines and 15 Chick-fil-A sandwiches, which Corkins said he planned to "smother" into the faces of the people he planned to kill, court records showed .
At about the time of the shooting, the head of the Chick-fil-A restaurant chain had announced his opposition to same-sex marriage.
Corkins had been a volunteer at a gay community center, the Post reported.