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.

Occupants were allowed back into the building shortly after 5 p.m., and the fire trucks left. Cambridge bomb technicians did not inspect the building.

At 5:25 p.m., a second bomb threat was phoned in to CPD headquarters.

Wrinn said HUPD units determined at the same time that the pungent odor was not a cleaning solvent but was coming from the granular material itself.

Less than half an hour later after being allowed back into the building, occupants were evacuated a second time.

Although the smell had been around for a while--a secretary reported smelling it around 4 p.m.--witnesses said it grew much stronger just before the second evacuation.

"My eyes started to tear," said Jeffrey E. DiIuglio, an instructor who was teaching his English as a Second Language course at the time.

He said occupants were allowed back into the building after the initial evacuation, but when they got to the third floor classroom, an odor overwhelmed the class.

"I couldn't believe they let us back into the building," DiIuglio said.

He characterized the smell as "ammonia-like."

Other witnesses, students in DiIuglio's class, reported the smell was "sweet" but "not quite fire."

An additional CFD engine responded the second time along with University technicians.

"Three HUPD officers, the custodian and one student complained of being irritated by the smell," Wrinn said.

The officers and the custodian were taken to UHS complaining of eye and respiratory problems.

The University brought in both the Triumvirate Environmental Company to clean up the site and the Covino Company to conduct substance tests.

"They are taking carpet samples, doing air tests and obviously they'll test a sample of the substance too," Wrinn said.

The building remained closed for the night as officials continued to investigate the incident.

"The reason the building is closed is because of the smell, not the second bomb threat," Wrinn said.

"Obviously, we won't reopen the building until everyone is sure it is safe," he added.