NASHVILLE — First came the warning, then came the blast, shattering the Christmas morning silence in the heart of the city’s tourist district.

Before dawn on Friday, Nashville police officers rushed to calls of gunfire on Second Avenue, a strip of honky tonks, restaurants and boot shops. Instead of gunfire, they found an R.V., blaring a strange and unsettling message: There was a bomb. It would detonate in 15 minutes.

When the R.V. did explode, it sent plumes of smoke billowing above the city, blew out windows in shops and offices for several blocks, left three people hospitalized — and Nashville shaken.

Police said the explosion was deliberate. It was also deeply unsettling, coming in an area that draws thousands of people nightly. But who set it off and why remained unknown as officials began to make sense of the blast.