Chapel Hill, N.C. — A suspicious letter addressed to Democratic presidential candidate John Edwards' campaign headquarters prompted the evacuation of a Chapel Hill post office Thursday afternoon.

Workers at the Franklin Street Post Office discovered the letter, which apparently contained an unknown powdery substance, at about mid-afternoon, Chapel Hill police said.

The letter was undergoing tests early Thursday evening to determine what the powder might be, but post office authorities said they did not believe it was a threat to the public.

The Edwards campaign had no comment on the incident and deferred comment to Chapel Hill police.

In March, Edwards' campaign office, based in Chapel Hill, was evacuated after a staff worker opened a letter that contained a white powder. The FBI is still investigating that case.

White powder has been associated with anthrax since 2001 when five people died and 17 people became ill after being exposed to it in mailed letters and packages.

The substance was mailed to lawmakers on Capitol Hill and members of the news media in New York and Florida weeks after the Sept. 11, 2001, terrorist attacks.