Philadelphia police said as of early Wednesday morning that they had no idea who sent a threatening letter to the Barack Obama election headquarters at 15th and Christian streets.
The letter was sent by mail and detected by an Obama worker shortly before 5pm on Tuesday.
Police were notified and summoned the fire department, secret service, and FBI as a precaution.
Twenty people inside the building were temporarily evacuated. No one was hurt.
A substance found inside the letter was quickly determined to be brown sugar and nothing hazardous.
The FBI and US Postal Service were attempting to track down the sender of the letter which had a San Diego postmark.