A military bomb squad was called in to retrieve two suspicious boxes abandoned at the exclusive basement parking section, but the packages turned out to be empty.
The senators and their staff were made to wait for several minutes at a parking lot outside the building, and were allowed inside again only after the explosives experts gave the all-clear signal.
Sniffer dogs used to search the building for explosives came upon the two boxes and sat down beside them, indicating there was a bomb inside.
Air Force Sgt. Mario Diaz said an empty plastic bag found inside one of the boxes may have previously been used to hold bullets or explosives, and that lingering traces of the materials remained on the bag, alerting the dogs.
Col. Jaime Dimacali, director of the Senate security enforcement service, surmised that the bag could have been used to hold pyrotechnics or firecrackers.
Diaz refused to speculate on how the boxes ended up at basement parking.
A Senate maintenance employee, Rodolfo Cristobal, reported that he spotted the boxes while he was about to go home Monday afternoon.
It was still unclear why they were left there overnight.
Senate Minority Leader Aquilino Pimentel Jr. said the prevailing peace and order situation must really be so bad "so that the reaction of the dogs (to the boxes) causes disruption (of legislative proceedings)."
Pimentel, along with Sen. Robert Barbers, said the matter should be investigated as there may be some people out to create trouble and take advantage of threats by disgruntled elements of society. — Aurea Calica, Jose Aravilla