Authorities had previously questioned Brandon Scott Hole and seized a weapon while the FBI was warned he might "suicide by cop", according to multiple reports.
Indianapolis Metropolitan Police Department’s deputy chief of criminal investigations Craig McCartt confirmed at a Friday afternoon press conference that authorities seized a gun from the suspected shooter in 2020.
The FBI's special agent in charge, Paul Keenan, separately confirmed that agents questioned the 19-year-old after his mother warned her son might "suicide by cop", according to Associated Press.
While they questioned Mr Hole about items found in his bedroom, no crime was identified and the FBI did not elaborate on what those items were.
Mr McCartt said police found Mr Hole, dead with a self-inflicted gunshot wound, inside the FedEx building where he last worked in the fall of 2020.
"FedEx officials have confirmed that Mr Hole was a former employee of the facility," he said.
After arriving at the facility on Thursday night, Mr Hole got out of his car and quickly began shooting randomly with a rifle, Mr McCartt said.
Four people were killed outside and another four inside, with several more injured. Police had still not notified all the victims' families on Friday.
"There were at least 100 people in the facility at the time of the incident, many were changing shifts and were on their dinner break," Mr McCartt said.
Search warrants have been executed at "multiple locations" connected to Mr Hole, who could not say whether the rifle used in the mass shooting was obtained legally or illegally.
Initial investigations showed there were no arguments or interactions before Mr Hole began shooting randomly for about two minutes, Mr McCartt said.
“There was no confrontation with anyone that was there, there was no disturbance, no argument, he just appeared to randomly start shooting and that began in the parking lot. And then he did go into the building, into the facility for a brief period of time," he said.