A source in the Philadelphia School District tells Action News that the message stated that some sort of device would go off Monday at noon at district headquarters 440 N. Broad Street.
However, a search of the building concluded on Sunday night with nothing found. On Monday morning, police were screening people who were entering the building and checking IDs.
A police source says that one 'unknown' item was observed and was cleared by the bomb disposal unit.
Computer forensics experts working on the case. Police say it could take a couple of days, but they expect to make an arrest.
The school district received the email over the weekend. School officials said on Sunday that the email threatened the 'life and safety' of staff and Superintendent Arlene Ackerman.
However, by Monday, police said there was no threat to Ackerman's personal safety. Instead, the writer threatened the building and after citing 'Ackerman's treatment of some teachers.'
A source tells Action News it read, in part "...enough is enough... a clear message has to be sent... tears will be shed."
Action News caught up with Ackerman on Monday morning.
"I'm more concerned about the people in this building and that the threat was directed at the building," Ackerman said.
"Whenever change happens it's inevitable that you are going to make people unhappy. You just hope they don't follow through on crazy actions or thoughts," she continued.
Ackerman said she had never received a threat like this during her time in Philadelphia, saying "This is the worst that I've seen."
"It was pretty disturbing."
The district says Ackerman does travel with her own driver and always has personnel on hand adept at keeping her safe.
The bomb squad, with the help of specially trained dogs, searched the headquarters top to bottom on Sunday.
Still, school district employees are being urged to report anything suspicious.