A bomb threat at the state Capitol prompted an evacuation of the Senate, House and executive tower Friday morning.

State and city police bomb units with explosive-sniffing dogs were called in to search the buildings, Department of Administration spokesman Alan Ecker said.

Nothing was found in the Executive Tower or the Old Capitol and employees were allowed to return to those buildings after a three-hour evacuation, Ecker said.

The House and Senate remained evacuated while bomb units searched those adjacent buildings. The Legislature was not in session Friday.

Despite the evacuation, Napolitano and other state officials held a ceremony in a nearby outdoor plaza to honor the victims in Monday's massacre of 32 students and faculty members at Virginia Tech.

The evacuation was ordered after a telephone caller told the Capitol Police Department at 7:21 a.m. that he would detonate a bomb in the Capitol, Ecker said. "All threats are taken very seriously and this is the procedure that the law enforcement felt was in the best interest of public safety."

State agencies in nearby office buildings were not affected by the evacuation, Ecker said.