A bomb squad had been called to the headquarters at 700 All America Way and the building closed around 12:30 p.m. after a citizen brought ammunition and grenades to the headquarters lobby for disposal.
The building was evacuated at that time, but workers are now back inside.
"The emergency has been mitigated," the department said.
Sunnyvale Communications is also back in service handling 911 calls, the department said. The 911 services had been moved to County Communications earlier, the agency said.