A trace of the two guns found by investigators at the scene revealed suspect Brandon Scott Hole, 19, of Indianapolis, legally bought the rifles last July and September, Indianapolis police said on Saturday.

They did not say where Hole bought what they described as "assault rifles", citing the ongoing investigation, but said he was seen using both rifles during the shooting.

Details about the weapons' make, model and calibre won't be released until the investigation is complete, said Genae Cook, a spokesperson for the Indianapolis Metropolitan Police Department.

Authorities said Hole shot and killed eight people, four of them from the city's Sikh community, at the FedEx facility late on Thursday before killing himself.

The FBI said agents had questioned Hole last year after his mother called police to say her son might commit "suicide by cop".

Officers seized a pump-action shotgun from Hole's home after responding to the call, according to a police report. Police said the gun was never returned to him.

Republican Senator Todd Young called on Sunday for more mental health services across all levels of government.

"We know that we have a Hoosier family who cried out for help, knowing they had a child who required mental health treatment," he said.

"We know we have members of our law enforcement community who, for a period of time, responded to that call for help. And we know that in the end, that wasn't enough."

Young questioned whether Indiana's red flag laws "were actually enforced" to prevent the shooting.

Mark Bode, a spokesperson for Indianapolis Mayor Joe Hogsett, said on Sunday his office also "continues to monitor closely the findings of the ongoing investigation, and what breakdowns in the red flag law processes may have come into play."

Indiana has had a "red flag law" allowing police or courts to seize guns from people who show warning signs of violence since 2005.

It became one of the first states to enact such a law after an Indianapolis police officer was killed by a man whose weapons had to be returned despite hospitalisation months earlier for a mental health evaluation.

The law is intended to prevent people from purchasing or possessing a firearm if they are found by a judge to present "an imminent risk" to themselves or others.

Authorities have two weeks after seizing someone's weapon to argue in court that the person should not be allowed to possess a gun.

Officials have not said whether Hole's case was brought before a judge. Michael Leffler, a spokesperson for the Marion County Prosecutor, said on Sunday the office is "looking into this matter".

If Hole had a court hearing and prevailed, state law indicates the shotgun would have been returned to him.

If a judge ruled him dangerous or incompetent, he should have been barred from buying another.

Australian Associated Press