Many press accounts have referred to her as a security guard, which sounded to me like a professional guard; but the pastor of New Life Church reports that she was a volunteer. Thanks to Call Me Ahab and David Hardy (Of Arms and the Law) for the pointer.
UPDATE: Thanks to the commenters for passing along more information — the woman’s name is Jeanne Assam; she was apparently a Minneapolis police officer for several years in the 1990s.