The University of Arizona Police Department evacuated about 100 students and staff from the Physics and Atmospheric Sciences building Wednesday night after a suspected natural gas leak.

A UA staff member inside the building called UAPD shortly after 5 p.m., reporting “”an odor out of the ordinary,”” Officer Mario Leon said at the scene.

UAPD then called the Tucson Fire Department to the scene to investigate, and the building was evacuated at 5:37 p.m., said Officer David Lopez.

After a TFD sweep of the building using Hazardous Materials Response Unit equipment found regular gas levels, authorities deemed the building safe for re-entry at about 6:15 p.m., Leon said.

TFD attributed the unusual odor to an unnamed professor, who was working with vacuum oil, a material whose smell could easily be mistaken for natural gas, TFD officials said.

All through traffic on Fourth Street was blocked during the incident, Lopez said.

Biology freshman Mohamed Hussein was in the building attending his mathematics class when the building was evacuated.

“”We thought there was some kind of threat going on,”” he said.

All classes continued as usual following the false alarm.