A suspect with mental health issues who allegedly killed four people in a Tennessee fast-food outlet on Sunday had been previously stripped of his right to own firearms and left a trail of red flags.
Questions are being asked about how Travis Reinking, 29, was apparently able to get his hands on the weapons.
Police said the AR-15 rifle he allegedly used in the attack at a Waffle House in a Nashville suburb had been confiscated from him just last year.
The AR-15 is a high-powered, semi-automatic weapon that has been used in a string of previous US mass shootings.
Mr Reinking, originally from Illinois, was already known to authorities after he was arrested outside the White House last year.
Secret Service officers detained him in July 2017 and charged him with unlawful entry after he crossed a security barrier.
He told Secret Service agents he "was a sovereign citizen and has a right to inspect the grounds" and that he wanted to meet President Donald Trump, according to a police report.
Following that arrest, authorities revoked Mr Reinking's Firearm Owners' Identification card.
They also confiscated four legally owned guns, including the AR-15 that was used in the Waffle House attack, police said on Sunday.
But authorities said the guns were returned to the suspect's father, Jeffrey Reinking, after he asked to keep them.
Sheriff Robert Huston in Tazewell County, Illinois, told a news conference on Sunday the father had promised he would "keep the weapons secure and out of the possession of Travis".
It was unclear how Travis Reinking apparently reclaimed the guns, said Sheriff Huston.
But Nashville Police spokesman Don Aaron said Jeffrey Reinking "has now acknowledged giving them back".
Sheriff Huston described Travis Reinking as a delusional man with mental health issues who claimed to hear voices and thought people were stalking him.
In August last year, Mr Reinking told police in Tazewell County that several dozen people were hacking into his computer and phone, according to arrest records.
He also complained people were "barking like dogs" outside his home.
In the last few years Illinois police have responded to other incidents involving Mr Reinking, arrest records show.
In June 2017, he threatened an employee at his family's construction crane business with an AR-15, then drove to a nearby swimming pool in Tremont.
After arriving there, police say Mr Reinking barged into the public pool, before diving into the water wearing a pink dress and exposing himself to lifeguards.
The rifle stayed in the vehicle on that occasion, according to a police report.
When his father was contacted by police after that incident, he told them he had earlier puts the guns out of his son's reach.
The police report states the father had told investigators "awhile back he took 3 rifles and a hand gun away and locked them up when Travis was having problems", reports the Tennessean.
But the report adds that after Jeffrey Reinking moved to another US state he returned the firearms to his son.
Police then told Mr Reinking's father "he might want to lock the guns back up until Travis gets mental help which he stated he would".
In May 2016, officers detained Travis Reinking in the car park of the drug store CVS after his parents - who were with him at the time - called police, arrest records show.
Police said Mr Reinking had claimed pop singer Taylor Swift was stalking him, and had hacked his Netflix account.
"Travis believed everyone including his own family and the police are involved," the officer said in the report.
"Travis stated he did not want to hurt Taylor Swift or anyone else, he only wanted the harassment to stop."
Mr Reinking told the officer that the singer had told him to meet her at a branch of Dairy Queen, a US ice cream chain, but when he showed up she ran away.
Police believed Travis Reinking might be suicidal, and managed to convince him to be taken to hospital for a mental health screening.
"Travis is hostile toward police and does not recognize police authority," an officer wrote in that incident report, according to WTVR-TV.
"Travis also possesses several firearms."