The shooter who killed six people at a school in Nashville, Tennessee, on Monday bought seven guns legally and hid them at home, police say.
Investigators say the suspect's parents felt the 28-year-old should not own weapons, and did not realise the guns had been concealed in their house.
Six people, including three children age nine, were killed in the attack at the Covenant School.
The suspect was under "doctor's care for an emotional disorder", police say.
They believe it was the school that was targeted rather than any particular individual but have not indicated a clear motive.
Tennessee has no laws that allow police to seize guns from violent suspects.
Despite the absence of such so-called red-flag laws, police said they would still have sought to have the weapons confiscated if authorities had had any warning that the suspect could have posed a threat.
The pupils killed in the attack were Evelyn Dieckhaus, Hallie Scruggs and William Kinney.
Three adult employees at the privately run Christian school also died: Cynthia Peak, 61, Katherine Koonce, 60, and Mike Hill, 61.
Ms Koonce, the head of the school, and Ms Peak, a substitute teacher, were both close friends with the Tennessee governor's wife.
Governor Bill Lee said his wife, Maria, had planned to meet Ms Peak the night after she was killed.
Police have spoken to the parents of the suspect, Audrey Hale, who was killed by police less than 15 minutes after the attack began.
Hale, who identified as transgender and was a former student at the school, was armed with three guns, including a semi-automatic rifle.
The attack took place after the killer conducted surveillance of the premises, drew maps and wrote what police described as a "manifesto".
A police spokesman said the attacker did not target any person in particular, but did target "this school, this church building".
Hale's parents thought the suspect had owned only one gun, but that it had been sold.
They believed the suspect "should not own weapons", and were unaware the suspect "had been hiding several weapons within the house", said Nashville Police Chief John Drake on Tuesday.
The weapons were legally purchased from five shops around the city.
The killer "was under care - doctor's care - for an emotional disorder", Chief Drake said, without providing further details.
If there had been reports of suicidal or violent tendencies, he added, police would have sought to confiscate the guns.
"But as it stands, we had absolutely no idea who this person was or if [the suspect] even existed," he said.
Police received the first call about the incident at 10:13 local time (15:13 GMT) on Monday.
The suspect drove to the school in a Honda Fit and got into the building by firing through one of the doors, which were all locked.
Video later released by Nashville police shows the shooter opening fire to shatter glass panes on the front doors, then wandering the school's deserted corridors - at one point walking past a room labelled "Children's Ministry".
In the CCTV footage, the suspect is wearing what appears to be a protective vest and carrying an assault-style rifle in one hand, with a second weapon visible hanging from the left hip.
The suspect fired shots on the ground floor before moving to the building's second floor.
As police cars arrived, the shooter fired on them from the second floor, striking one vehicle in the windscreen.
"We believe there has been some training to have been able to shoot from a higher level," Chief Drake said.
He said the suspect had stood away from the glass to avoid being an easy target for police.
One officer was injured by broken glass. Police rushed inside and shot the suspect dead at 10:24, said Chief Drake.
President Joe Biden on Tuesday called for Congress to pass new gun control legislation.
"As a nation, we owe these families more than our prayers," he said. "We owe them action."
US Attorney General Merrick Garland was asked during a Senate hearing in Washington DC whether the attack will be investigated as a hate crime targeting Christians.
He said it was too early to say and that agents were still working to identify a motive.
The attack was America's 131st mass shooting so far this year, according to Gun Violence Archive, a non-profit that tracks gun violence data.
There have been 15 mass shootings at schools or universities in the US since the Columbine High School massacre in 1999, according to a database maintained by the Associated Press.