Authorities are trying to find out who sent a threatening letter containing a suspicious substance to one of Barack Obama's South Philadelphia campaign offices.

The office at 15th and Christian streets was evacuated for less than an hour after the discovery about 5 p.m. yesterday.

Police say a volunteer brought in the mail and opened a letter that contained the unknown substance. Officials evacuated the office, took the letter outside and called police.

A hazardous-materials crew responded, but investigators say initial tests determined that it was brown sugar.

Philadelphia police Capt. Mike Gillespie called the letter "somewhat threatening in nature."

An Obama spokesman did not return a telephone call from the Associated Press. *

- Associated Press