Anthony Quinn Warner, 63, has been named as the suspect.
The FBI said he blew up a camper van outside a telecoms firm, killing himself and injuring three others.
A motive is yet to be established. Warner’s criminal record only consisted of an 1978 marijuana-related charge.
On Monday the Tennessee Bureau of Investigation said Warner, who had worked in IT and had extensive experience with electronics, was “not on our radar”.
But a statement and report released by the Metropolitan Nashville Police Department (MNPD)on Tuesday shows local and federal authorities were made aware of Warner in August 2019.
What does the police report say?
The report recounts an incident that officers were called to at the Nashville home of Warner’s girlfriend Pamela Perry on 21 August, 2019.
Ms Perry’s lawyer, Raymond Throckmorton, asked police to visit her home after expressing concern about comments she had made.
When the officers arrived, they found Ms Perry sitting on the porch with two unloaded pistols nearby.
She related that the guns belonged to a ‘Tony Warner’ and that she did not want them in the house any longer,” Nashville police said in a statement.
The police force said Ms Perry eventually agreed to be taken by ambulance for a psychological evaluation.
But prior to this, Ms Perry told the officers that her boyfriend was “building bombs in the RV (recreational vehicle) trailer at his residence”, the police report says.
The report says Mr Throckmorton also told officers that Warner “frequently talks about the military and bomb making”.
The lawyer, the report says, “stated he believes that the suspect knows what he is doing and is capable of making a bomb”.
In an interview with The Tennessean newspaper , Mr Throckmorton said he urged police to investigate Ms Perry’s allegations, telling them he feared for her safety.
The MNPD said it did send officers to Warner’s home – about 2.4km (1.5 miles) from Ms Perry’s – on 21 August 2019. They knocked several times and saw the RV at the back of Warner’s home.
But Warner did not answer, so the officers left because “they saw no evidence of a crime and had no authority to enter”.
The police department’s hazardous devices unit was given a copy of the report.
A day later, Nashville police asked the FBI to run a background check on Warner.
The FBI later reported that it had “no records of Warner at all” and checks for any military connections “were all negative”.
“At no time was there any evidence of a crime detected and no additional action was taken,” Nashville police said.
“No additional information about Warner came to the department’s or the FBI’s attention after August 2019.”
What do we know about the explosion?
Police responded to reports of gunshots just before 06:00 local time (12:00 GMT) on 25 December in an area of the city known for its restaurants and nightlife.
Shortly afterwards, they found a camper van broadcasting a warning message to leave the area.
Police said the van had also broadcast the 1964 hit song Downtown by British singer Petula Clark.
The van exploded a few minutes later and Warner is the only person known to have died.
It blew up outside a building belonging to the telecoms giant AT&T, disrupting communications systems in Tennessee and four other states
Police are now trying to establish what motive Warner may have had for carrying out the attack.
The FBI is still combing downtown Nashville for clues, with agents expected to complete the crime-scene investigation by Friday.