It happened early Wednesday morning on the 1300 block of Atlantic Avenue.
According to police, this is the third time this week that fires have been set outside the building.
This blaze was the most damaging, investigators say. County officials announced that the Bacharach Boulevard employee entrance is closed until further notice.
"We have cameras on our buildings and we are reviewing the videotapes in consultation with the county's Department of Public Safety, Fire Marshal, and the County Prosecutor. We have our suspicions as to who is responsible," said County Executive Dennis Levinson. "The fires appear to be deliberate acts of arson."
No injuries were reported at the scene. Police are investigating the incident and reviewing surveillance footage from around the area.
"We have a duty to protect the safety of our employees, our customers, and our properties. We will investigate and prosecute so these unlawful acts do not go unpunished," Lavinson added.