PHILADELPHIA (CBS) – Police officers surrounded the Philadelphia school administration building all day Monday to ensure safety after an emailed threat came in Saturday night.

The threat targeted Superintendent Arlene Ackerman and her staff, saying the attack would come mid-day Monday. Security blanketed the superintendent as she arrived at the school administration building Monday morning.

“This is the most serious I have seen,” Ackerman says of the threat. “Most are just phone calls that are intercepted, but this one was disturbing.”

Security guarded the entrance, checking bags and patrolling the hallways.

Blaine Wylie brought his niece to the building to apply for a job. He noticed the added security immediately.

“Once they directed her where she had to go, security was right on it,” says Wylie.

In the parking lot, police K-9s searched vehicles for anything suspicious, but police say they found no evidence of any explosives. Everyone entering the building had to show identification and be ready for extra security measures.

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Superintendent Ackerman says the threat will not stop the district from its work, as administrators try to sort out a budget shortfall nearing $500 million.

“Not everyone is happy with what’s happening, but I am doing what’s right in my heart,” says Ackerman.

Police say they have not made an arrest yet, and they do not know who sent the email. They say that remains under investigation.

The district says they will likely have added security all week to ensure everyone’s safety.

Reported by Oren Liebermann, CBS 3