WASHINGTON (AP) — A man who had been a volunteer at a community center for gays walked into the lobby of a conservative political organization, assailed the group's work, pulled a gun and opened fire.

But the shooting Wednesday morning at the downtown Washington headquarters of the Family Research Council left only one person injured: the security guard. And after being struck in the arm, he helped wrestle the gunman to the floor, thwarting an attack that police fear could have turned deadly.

"The security guard here is a hero, in my opinion," D.C. Police Chief Cathy Lanier said of Leo Johnson, who was conscious and in stable condition at an area hospital after the shooting.

Police and the FBI were investigating why the armed man, identified as 28-year-old Floyd Lee Corkins II of Herndon, Va., entered the front lobby of the conservative group, argued with the guard and opened fire. But one law enforcement official, speaking on condition of anonymity, said the suspect made a negative reference about the group's work and what it stands for before shooting.

Corkins was being held on a charge of assault with a deadly weapon, the FBI said in a news release Wednesday night. Authorities were interviewing him and canvassing his neighborhood in Herndon, located about 24 miles from the nation's capital.

Television news footage showed the suspect, a large man with a shaved head and an unbuttoned striped shirt, being led to a car with his hands restrained. It was not immediately clear if he had a lawyer.

Though authorities did not publicly reveal a motive, advocacy groups across the ideological spectrum condemned the violence, with some casting it as a hate crime. President Barack Obama was concerned about the well-being of the guard, a White House spokesman said, and Republican presidential candidate Mitt Romney also said he was appalled.

"Today's attack is the clearest sign we've seen that labeling pro-marriage groups as 'hateful' must end," Brian Brown, the president of the National Organization for Marriage, said in a statement.

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The Family Research Council, headquartered in a busy downtown tourist district, strongly opposes gay marriage and abortion and says it advocates "faith, family and freedom in public policy and public opinion." The group maintains a powerful lobbying presence on those causes, testifying before Congress and reviewing legislation. Its president, Tony Perkins, said the group's main concern was with the wounded guard.

Corkins, who had been volunteering recently at a community center for lesbian, gay, bisexual and transgender people, made a negative comment about the organization's activity before the shooting, but the reference was not specific, one of the law enforcement officials said. Two law enforcement officials said Corkins was carrying sandwiches from Chick-fil-A, a fast-food chain whose president's public opposition to same-sex marriage recently placed the restaurant at the center of a national cultural debate.

The officials spoke on condition of anonymity because the investigating is continuing.

Johnson, who was taken to the hospital Wednesday, was expected to survive.

"The security guard did a phenomenal job, above and beyond what he was supposed to do in this particular situation," said Jacqueline Maguire, a spokeswoman for the FBI's Washington field office.

Corkins had been volunteering for about the past six months at The DC Center for the LGBT Community, said David Mariner, executive director of the community center, in Northwest Washington. He usually staffed the center's front desk on Saturdays, and his most recent shift was about two weeks ago.

"He always struck me as a kind, gentle and unassuming young man. I'm very surprised that he could be involved in something like this," Mariner said.

Authorities seized Corkins' car at a northern Virginia Metro station, and were going door-to-door interviewing neighbors, several of whom spoke highly of the family.

"They were always so sweet and so nice," said Stephanie Meyer, who lives a few doors down. "They are awesome people. We never had any issues."

According to a U.S. Defense Department official, Corkins is not a member of the Air Force, but he may have lived at Andrews Air Force base in some other capacity in the past, possibly as a dependent or family member. The official spoke on condition of anonymity because the official was not authorized to provide personal information.

Amy Biondi and her husband Steve were visiting Washington from Long Island with their daughter and a friend and tried to ask officers for help with a parking meter when they were told there was a situation they had to deal with. The door to the FRC was opened, and an officer could be heard repeatedly shouting, "Put the gun down, put the gun down."

"Next thing you know there are police officers swarming the area," said Biondi, 45, a massage therapist from St. James, N.Y.

The family didn't get a close look inside, but they said the man that officers were talking to seemed to comply immediately.

Groups aligned with conservative causes lambasted the shooting, but so did a coalition of about two dozen organizations promoting gay, lesbian and transgender rights, which said it rejected and condemned the attack.

White House spokesman Jay Carney said the president, who was traveling in Iowa Wednesday, was informed of the shooting shortly after 1 p.m.

"The president expressed his concern for the individual injured in the shooting and his strong belief that this type of violence has no place in our society," Carney said.

Romney said in a statement that he was appalled. "There is no place for such violence in our society," he said. "My prayers go out to the wounded security guard and his family, as well as all the people at the Family Research Council whose sense of security has been shattered by today's horrific events."

The headquarters of the FRC is in the city's bustling Chinatown neighborhood, near the Verizon Center, the Smithsonian National Portrait Gallery and several museums, restaurants and shops.

In the past month, the FRC has forcefully defended Chick-fil-A President Dan Cathy for his remarks in opposition to gay marriage.

Mariner said he did not know Corkins well or have any conversations with him about the Chick-fil-A controversy or other political issues of interest to the gay community.

"I really only talked to him about volunteering, so I couldn't say anything about anything else," Mariner said.

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Associated Press writers Ben Nuckols and Lolita Baldor in Washington and Matthew Barakat in Herndon, Va., contributed to this report.