They've got muck; we've got rakes. TPM Muckraker
The unnamed suspect, described by one witness who saw him in handcuffs as a middle-aged man, is currently in custody and being interviewed by authorities. Officials from the Metropolitan Police Department; the Bureau of Alcohol, Tobacco, Firearms and Explosives; and the FBI are on the scene.
Authorities are looking into the suspect's motivations in order to determine whether to treat the incident as domestic terrorism, FBI spokeswoman Jacqueline Maguire told reporters. If it is not domestic terrorism, the case will likely be handled by MPD (though federal prosecutors in the U.S. Attorney's would still handle the prosecution).
Best known for its stance against gay marriage and sponsoring the annual Values Voter Summit, the organization was formed by James Dobson in 1981.
Video from an FBI press conference: