Police spokesman Jeff Thomason said Oakland police and the Alameda County Sheriff's bomb squad have been dispatched to the building to investigate the situation.
Burris, who wasn't at his office Tuesday morning, said his staff became suspicious when a man came into his office, dropped off a medium-size box and left around 10:15 a.m. Tuesday.
"My staff was shook up," Burris said.
He said his staff members contacted federal Transportation Security Administration officials, who have an office in the building. TSA officials told them to leave the package alone and that it should be examined by a bomb squad.
The building, located Interstate Highway 880 between the 66th Avenue and Hegenberger Road exits, houses a number of businesses, including the Oakland Tribune.
Burris recently filed a $50 million lawsuit against BART on behalf of Oscar Grant, who was shot and killed by BART police officer Johannes Mehserle at the Fruitvale station in Oakland early on New Year's Day.
Thomason said he's not sure if the entire building has been evacuated or just a few floors.
