The Dallas-based communications giant said Saturday that two portable cell sites had begun operating in the downtown area, and workers were setting up others in the city and the region.
"At our facility, the focus of the restoration continues to be getting power to the equipment in a safe and secure way," the company said in a statement on Saturday. "Challenges remain, including a fire which reignited overnight and led to the evacuation of the building."
On-site teams working with safety and structural engineers have "drilled access holes into the building and are attempting to reconnect power to critical equipment," AT&T said. "Technical teams are also working as quickly as possible on rerouting additional services to other facilities in the region to restore service."
The damage occurred early Friday, not long after police responding to a report of shots fired encountered an RV blaring a recorded warning that a bomb would detonate in 15 minutes, Metro Nashville Police Chief John Drake said. Police evacuated nearby buildings and called in the bomb squad.
Authorities are unsure whether the site of the explosion -- near an AT&T building one block from the company's landmark office tower -- "was a coincidence, or if that was the intention," police spokesman Don Aaron said.
AT&T said the affected building is the central office of a telephone exchange, with network equipment in it.
The AT&T outages site showed service issues in middle Tennessee and Kentucky on Friday. Several police agencies reported that their 911 systems were down because of the outage, including Knox County, home to Knoxville about 180 miles (290 kilometers) east of Nashville.
The Federal Aviation Administration temporarily halted flights out of Nashville International Airport because of telecommunications issues associated with the explosion.
The FBI will be taking the lead in the investigation, agency spokesman Joel Siskovic said. Federal investigators from the Bureau of Alcohol, Tobacco, Firearms and Explosives were also on the scene. The FBI is the primary law enforcement agency responsible for investigating federal crimes, such as explosives violations and acts of terrorism.
President Donald Trump has been briefed, according to White House spokesperson Judd Deere. The U.S. Justice Department said Acting Attorney General Jeff Rosen was also briefed and directed all department resources be made available to help with the investigation.
Tennessee Gov. Bill Lee said on Twitter that the state would provide the resources necessary "to determine what happened and who was responsible."
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