A phoned-in bomb threat followed by a strange smell forced two separate evacuations of a building at the Harvard Extension School, cancelled classes and sent at least four people to University Health Services (UHS) yesterday.

At 4:37 p.m., the Extension School registrar's office at 51 Brattle St. received a call about explosives in the building.

The Harvard University Police Department (HUPD) ordered the building's 200 occupants evacuated, and the Cambridge Fire Department (CFD) responded to search the building.

Initial confusion at the scene led the Cambridge Police Department (CPD) to broadcast a report that an explosion had occurred at the building.

Police officers guarded the building's entrance as firefighters and three HUPD officers searched the building. CPD units shut down Brattle Street while worried building workers milled about across the street.

About the same time, a custodian discovered a "handful of granular rock-salt-like material" in the building, according to University spokesperson Joe Wrinn.

The custodian began to clean up the material with a paper towel.

Officials noticed a pungent odor in the air, but assumed it was from cleaning solvents the custodian was using, and they declared the building clear after not finding anything suspicious.