Nashville police officers were first called to a report of shots fired, police said. There was no evidence of shots fired, but "there were announcements coming" from an RV saying a potential bomb would detonate within 15 minutes, police said.
The recording only began playing a short time after police reported to the scene, a law enforcement official told ABC News.
Officers were working to evacuate nearby buildings when, around 6:30 a.m., the RV exploded, blowing out the windows of nearby buildings.
Human remains have been found at the scene of the explosion in downtown Nashville, multiple law enforcement sources told ABC News.
The remains have not been identified and it's unclear whether they're identifiable.
"We found tissue that we believe could be human remains," Metro Nashville Police Chief John Drake said at a press conference Friday evening. "We'll have that examined and we'll be able to tell you from that point."
The explosion knocked one officer to the ground. Another officer sustained temporary hearing loss.
Three people were taken to the hospital with minor injuries.
It's not known whether anyone was in the RV when it exploded, authorities said.
Nashville International Airport temporarily halted flights Friday afternoon after losing phone and internet in the air traffic control tower due to the blast.
AT&T said service was impacted in Nashville and surrounding areas because of damage to its facilities from the explosion.
Communications capabilities came back in the tower about an hour after the ground stop was issued and service resumed, the Federal Aviation Administration said.
Resident Buck McCoy said he heard gunshots before the explosion, which "blew everything all over the entire apartment."
"All the windows came in from the living room into the bedroom," he told ABC News. "There was glass everywhere from the windows. There wasn't really any part of the apartment that wasn't damaged or pushed or moved or affected by the explosion ... the whole apartment was just completely a mess."
"My building is pretty tall and pretty strong so it had to have been a very strong explosion to blow out all those windows," he said.
Several buildings suffered structural damage, authorities said.
McCoy said he "started to go downstairs, saw some people in the hallway -- they were in shock, you know, they're crying."
He then reached the street which he said looked "like a movie ... it was just surreal."
The debris field extends for at least a few blocks. Streets around the exploded vehicle have been closed down.
In the search for the suspect, there are "investigative leads to be pursued" and "technical work that needs to happen," the FBI said.
"We will find out what happened here," the FBI said.
The FBI, the lead investigating agency, asks anyone with information to submit a tip on the FBI website.
Agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives have also responded.
President Donald Trump, President-elect Joe Biden, acting Secretary of Homeland Security Chad Wolf and acting Attorney General Jeff Rosen have been briefed on the incident.
Authorities said they are not aware of other imminent danger. K-9 officers swept the downtown area as a precaution, police said.
Mayor John Cooper also instituted a curfew for the area affected by the explosion from 4:30 p.m. on Friday to 4:30 p.m. on Sunday.
ABC News' Mina Kaji contributed to this report.