Fox News:

The gunman, identified as 28-year-old Floyd Lee Corkins II, entered the lobby of the organization's Chinatown headquarters around 10:45 a.m. and expressed disagreement with the conservative group's policy positions, sources tell Fox News.

... The suspect "made statements regarding their policies, and then opened fire with a gun striking a security guard," a source told Fox News.

Sources also said the gunman may have been carrying a bag from Chick-fil-A, the embattled fast-food restaurant whose president came under fire from gay activists after he said he did not agree with same-sex marriage.

Sources told Fox News that after guard took away his gun, the suspect said, "Don't shoot me, it was not about you, it was what this place stands for."

Authorities were treating the attack as a case of domestic terrorism, although James McJunkin, the head of the FBI’s Washington Field Office, said authorities do not yet know the gunman's motive.

... 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, which is in Northwest Washington. He usually staffed the center's front desk on Saturdays, and his most recent shift was about two weeks ago.