An armed intruder, spouting opposition to social conservatism, walked into the Washington headquarters of the Family Research Council on Wednesday and shot a security guard before the wounded guard and others wrestled him to the floor and subdued him until police arrived, authorities said.

They identified the suspect as Floyd Lee Corkins II, 28, of Herndon, who has a master’s degree from George Mason University’s College of Education and Human Development. Corkins was being held late Wednesday night on a charge of assault with a deadly weapon.

The guard, identified by the Family Research Council as Leo Johnson, was shot in the arm before or during the incident in the lobby, D.C. Police Chief Cathy L. Lanier said. She credited Johnson with preventing a tragedy by stopping the man, who was carrying a 9mm handgun, from reaching the upstairs offices in the group’s six-story building at Eighth and G streets NW.

“The security guard here is a hero, as far as I’m concerned,” Lanier said outside the building, across from the National Portrait Gallery and a block from Verizon Center, an area crowded with tourists on a sunny morning.

“He did his job,” Lanier said. “The person never made it past the front.”

Johnson was hospitalized in stable condition, authorities said. No one else was injured.

The FBI said it was evaluating evidence to determine whether to charge Corkins with a federal crime, such as attempting a terrorist act. Authorities said the decision will hinge on what the FBI concludes was the assailant’s motive for entering the building with a loaded Sig Sauer semiautomatic.

Corkins is being held on a local charge. But a federal official said FBI agents would work through the night and they expected to charge Corkins in federal court as soon as Thursday.

Officials said they were interviewing Corkins and searching his vehicle. They also were at his Herndon home Wednesday night interviewing his family and neighbors.

The Family Research Council, a conservative lobbying group, deals in issues of faith, family and freedom, its Web site says. The organization opposes abortion and euthanasia, among other practices, and says it considers homosexuality to be a sin.

Social-issues groups on the left and right of the political spectrum and both presidential candidates weighed in on the shooting in e-mails and public statements.

Gary Bauer, a former president of the Family Research Council and now head of American Values, cited “a disturbing level of intolerance and hate aimed at those who share traditional values.”

“Men and women of faith must not be intimidated into silence,” Bauer said.

Speaking for several lesbian, gay, bisexual and transgender organizations, the Gay & Lesbian Alliance Against Discrimination said: “We utterly reject and condemn such violence” and “wish for a swift and complete recovery for the victim of this terrible incident.”

Two law enforcement officials, speaking on the condition of anonymity because the investigation is ongoing, said the gunman entered the lobby carrying a satchel, with a bag from a Chick-fil-A restaurant inside. The Atlanta-based fast-food chain has been embroiled in controversy in recent weeks after its president spoke out against same-sex marriage. The Family Research Council also opposes such unions.

“Today’s attack is the clearest sign we’ve seen that labeling pro-marriage groups as ‘hateful’ must end,” said Brian Brown, president of the National Organization for Marriage.

Corkins had been volunteering at a U Street NW community center for gay, lesbian, bisexual and transgender people, according to officials there.

Michael Sessa, president of the D.C. Center for the LGBT Community, said the FBI has spoken with staff members about Corkins’s time there, which had been about six months. Corkins would have performed administrative work checking in people at the front desk and would have undergone a background check, Sessa said.

“We are all shocked,” he said.

David Mariner, the center’s director, said in a statement: “We condemn such violence in the strongest terms possible. We hope for a full and speedy recovery for the victim, and our thoughts are with him and his family.”

Allan P. Chan, 28, a former George Mason student, said he met Corkins at a campus gym about six years ago. They worked out together, lifting weights, and began to socialize and watch television together. Chan described Corkins as secretive and somewhat odd. Corkins’s Facebook page included no photos, not even his own, and he displayed an intense interest in the 19th-century German philosopher Friedrich Nietzsche.

“He was a fanatic of Nietzsche,” Chan said.

Corkins apparently traveled to the District on a Metro train Wednesday morning after parking his car at the East Falls Church station, authorities said.

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.

Gina Dalfonzo, a policy analyst for the organization a decade ago, recalled him as “a pretty imposing guy” with “a professional demeanor.”

The council’s president, Tony Perkins, declined to discuss the incident in detail, referring inquiries to the police and FBI. “Our first concern is with our colleague who was shot,” Perkins said in a statement. “Our concern is for him and his family.”

Officials said Corkins’s gun was bought legally within the past several weeks.

“Our initial investigation has found no criminal activity associated with the gun or its purchase,” said Richard Marianos, the agent in charge of the D.C. office of the federal Bureau of Alcohol, Tobacco, Firearms and Explosives.

James McJunkin, the head of the FBI’s Washington Field Office, said investigators were seeking to determine why the gunman entered the building.

“We don’t know enough about him or his circumstances . . . or his mental state or what he was doing or thinking of doing,” McJunkin said. “So we’re going to try to sort this all out, pull the evidence together, do all the interviews we can.”

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.