After an R.V. packed with explosives detonated in downtown Nashville on Christmas morning, law enforcement officials assured the public that they had repeatedly attempted to follow up on a 2019 tip that the perpetrator, Anthony Q. Warner, had been building bombs in a vehicle parked outside his house.

“Legally, there was nothing else that could have been done,” the Nashville police chief, John Drake, told reporters after the explosion.

But a report released on Wednesday found no documentation of any attempts to reach Mr. Warner after officers knocked on his door and got no response. The report added that the police should have acted more aggressively, keeping closer tabs on Mr. Warner and trying to establish the probable cause needed to search his home. Instead, the case sat open for months, as Mr. Warner stewed in conspiracy theories and plotted the attack.

The report emerged from an inquiry into the police investigation conducted by a panel that included senior officials from the Metropolitan Nashville Police Department and others from outside the agency. The panel concluded that there was “no way to know for sure if the suicide bombing on December 25, 2020, could have been prevented.” Yet it identified ways in which the Police Department had failed to conduct a robust investigation into the earlier warning.