Indianapolis police say a gunman who opened fire at a FedEx warehouse killing eight people had legally bought the two assault rifles he used, despite having a shotgun confiscated months earlier.
Brandon Hole's mother had warned about his mental state last March and police had taken a gun he owned away from him.
But he was subsequently able to buy two assault rifles in July and September.
On Saturday his family apologised for the "pain and hurt" his actions caused.
The 19-year-old former FedEx worker opened fire at the warehouse on Thursday before killing himself minutes before police arrived.
Four of the dead have been identified as members of the local Sikh community. Other victims include two 19-year-olds, a university graduate and a father.
Police have not yet identified a motive for the attack.
Hole's mother called police last March and told them she feared her son might try to commit "death by cop", Paul Keenan, special agent in charge of the FBI's Indianapolis field office, said.
Officers arrested him and took away his shotgun, records quoted by the Indianapolis Star newspaper show.
The FBI then interviewed him a month later but did not find evidence of a crime and did not identify Hole as following extremist ideology, Agent Keenan said.
Hole used both assault rifles in the attack, police said.
He fired randomly at people in the FedEx facility's parking lot, fatally wounding four, before entering the building and killing four more people.
Indianapolis police said they could not give details about where he had bought the rifles as the investigation was ongoing.
President Biden has called the Indianapolis shooting and other recent mass shootings a "national embarrassment".
Earlier this month he announced his first steps to tighten gun controls. They include efforts to set rules for certain guns, bolster background checks and support local violence prevention.
What are the rules for buying guns in Indiana?
Under US federal law licensed gun dealers must carry out a background check on buyers wanting a firearm.
In Indiana licensed gun dealers must use the FBI's National Instant Criminal Background Check System (NICS), which checks that the buyer does not have a criminal record or is otherwise ineligible to buy a gun. Its website says 300 million checks have been carried out, resulting in 1.5 million people being denied a gun.
However this does not apply to unlicensed private sellers. Indiana and most other US states do not require a background check to buy a gun - including an assault rifle - from an unlicensed seller, according to Giffords, which campaigns against gun violence.
Giffords says nearly a quarter of US gun owners bought their most recent gun without a background check. The loophole lets guns "easily find their way into the hands of illegal buyers and gun traffickers, dramatically increasing the likelihood of gun homicides and suicides", it says.
President Biden has said he wants to bring in universal background checks, which would apply to all gun sales. He also wants to ban the manufacture and sale of assault weapons, regulate existing assault weapons and introduce a buy-back scheme.
However the current make-up of Congress means enacting new gun laws will be difficult. The US Senate is currently split 50-50 between Democrats and Republicans, with Vice-President Kamala Harris holding the deciding vote. But current Senate rules mean that in practice, 60 votes are needed to pass legislation, meaning some Republican support is required. Republicans have blocked significant gun control laws in the past.
The right to bear arms is protected by the Second Amendment to the US Constitution and many people see gun control laws as infringing on this constitutional right.