Reuters:

A portion of a sixth-floor ABC News office housing "Good Morning America" was closed down for five hours Friday afternoon after an employee found a letter containing an unidentified white powder.

A portion of a floor in the building at 147 Columbus Ave. was shut down after 1 p.m. when the unnamed employee found the letter. Addressed to "Good Morning America" weatherman Sam Champion, the letter mentioned anthrax.[..]

ABC News said police were questioning a "person of interest" in connection with the case. The news organization said that "Good Morning America" operations would resume Friday and continue throughout the weekend. Even though tests for anthrax came back negative, the area would be thoroughly cleaned.

Although there was some discussion that Champion as to why was targeted, a former studio temp admitted to sending the letter and is being questioned.