The bag was placed near a planter and covered with green material just before it was detonated.
A suspicious device has been reported at the City Administration Building in National City.
A man brought a bag into the police station at the corner of 12th and A Streets around 11: 35 a.m. claiming he found it, officials said.
Once they looked inside, officials said the bag was holding nitroglycerin.
The bag was put outside and the bomb squad detonated it safely.