The Philadelphia School District said Sunday that it had received an e-mail threatening the life of Superintendent Arlene C. Ackerman.

The district said the threat was highly unusual because it specified that the attack would occur midday Monday at the district's North Broad Street headquarters.

Jamilah Fraser, a district spokeswoman, declined to discuss details of the e-mail, citing a pending investigation, but said it referenced several complaints about the district.

The matter has been referred to the Department of Homeland Security.

A police sweep of district headquarters Sunday turned up nothing. The district said that it would beef up security around Ackerman and that police would make another sweep of the building Monday morning.