Raw, radiological material that terrorists could use to build a dirty bomb was secured in a Philadelphia school this week.

On March 11, the U.S. National Nuclear Security Administration (a semiautonomous branch of the Energy Department) and Philadelphia’s Temple University announced they had secured a device containing cesium 137 — one of more than two dozen such elements used in medicine and industry that could be turned into a dirty bomb.

“This operation is part of NNSA’s broad strategy to keep dangerous nuclear and radiological material safe and secure by enhancing our nation’s security,” NNSA deputy administrator Anne Harrington said.

A terrorist dirty bomb attack using domestic radioactive sources bomb may seem preposterous, yet the NNSA has identified more than 2,700 vulnerable buildings with high-priority radiological material in the United States alone.