About 10:45 a.m., the shooter arrived at the Family Research Council’s brick-and-stone building at 801 G St. NW and walked through the tall glass-and-metal front doors, under an archway chiseled with group’s motto: “Faith, Family, Freedom.” Johnson, the guard, confronted him in the lobby, Lanier said.

The building is not far from the Gallery Place Metro station in one of the busiest areas of the city, thick with restaurants, shops and museums.

In the lobby, the intruder began “making statements” in opposition to the Family Research Council’s social conservatism, a law enforcement official said.

In the struggle that followed, Johnson “did a phenomenal job,” FBI spokeswoman Jacqueline Maguire said. She and Lanier said that if the gunman had gotten past the lobby and into the offices, a mass shooting might have occurred.

Neither investigators nor the group’s spokesman, Darin Miller, would disclose personal information about Johnson, who has been a longtime presence in the building.

At the East Falls Church Metro station, where authorities said Corkins parked his car, investigators towed away two vehicles about 3 p.m. A white sedan was on the back of a flatbed tow truck with a green sedan that had a damaged front end.

Mark Berman, Magda Jean-Louis, Jennifer Jenkins, Justin Jouvenal, Allison Klein, Fredrick Kunkle, Del Quentin Wilber, Clarence Williams and Mihirs Zaveri contributed to this report.