The Franklin County Sheriff's Office says the explosion happened around 7:40 a.m. on the north side of the building at 14111 Capital Blvd., along the Wake County-Franklin County line.
Before noon, investigators say they found a second explosive device less than 20 feet away from the first device. Authorities removed that device from the building and disabled the device outside.
The explosive device that went off is described by officials as a small tube-like device that Sprint employee Chris McMillan was holding in his hand while in a third-floor break room.
"I think the best thing we can do right now is assure people that everything is OK, is under control. The building is as secure as it can be right now," says Franklin County Sheriff Robert Redmond.
Authorities used bomb-sniffing dogs to see if there were any other devices in the building.
According to Lt. Nelson Ross, McMillian is being treated for shrapnel wounds to the hand at WakeMed.
The explosion happened before most of the several hundred people who work in the building had arrived. About six peoplewere in the vicinity of the blast on the third floor.
Employees were sent home just after 11 a.m.
The building is an administrative facility for Sprint, andworkers there handle marketing, public relations, human resourcesand similar tasks primarily for Sprint's operations in NorthCarolina, and to varying degrees for Virginia, South Carolina andTennessee.