The man was taken into custody by the FBI and was being interviewed. Authorities did not identify the man or disclose where he was being interviewed.
Police and FBI officials said it's too early to know the circumstances of the shooting, which occurred around 10:45 a.m. at the headquarters of the Family Research Council, or whether it was connected to the group's activities.
"We don't know enough yet about him ... or mentally what he's thinking," said James McJunkin, assistant director in charge of the FBI's Washington field office.
The Family Research Council confirmed in a statement that the security guard was employed by the group.
"Our first concern is with our colleague who was shot today," the group's president, Tony Perkins, said in a statement.
The Family Research Council advocates conservative positions on social issues and strongly opposes gay marriage and abortion.
Perkins was an outspoken defender of Chick-fil-A President Dan Cathy's public stand against same-sex marriage, which made the fast-food chain a flashpoint in the nation's culture wars. The Cathy family foundation has funded the Family Research Council.
"He's taking a bold stand," Perkins said after Cathy's comments were reported. "Chick-fil-A is a Bible-based, Christian-based business who treats their employees well. They have been attacked in the past about their stand. But they refuse to budge on this matter, and I commend them for what they are doing."
Associated Press writer Ben Nuckols contributed to this report.