Vester Lee Flanagan, who shot dead two Virginia journalists on live TV, apparently sent a rambling fax to ABC News describing himself as a “human powder keg” shortly after the attack.
Vester Flanagan, who had been dismissed by the WDBJ7 TV, killed himself after a police chase.
In the fax, a man said to be Vester Flanagan describes suffering discrimination and bullying for being gay and black.
The White House said Wednesday’s attack showed the need for better gun control.
The two people killed were WDBJ7 reporter Alison Parker, 24, and cameraman Adam Ward, 27.
WDBJ7 manager Jeff Marks said: “I cannot tell you how much they were loved.”
He said Vester Flanagan was an “unhappy man” who had to be escorted from the WDBJ7 building after being dismissed in 2013.
Suddenly, shots rang out, and viewers saw the camera fall to the ground. Screams could be heard and the footage captured a brief glimpse of the gunman.
The station cut back to the studio – journalists would later have to continue broadcasting on the deaths of two of their colleagues.
Hours later, Vester Flanagan posted footage online of himself opening fire at close range, although this was later removed.
The woman who was interviewed at the time of the attack, Vicki Gardner of the Smith Mountain Lake Regional Chamber of Commerce, is in a stable condition in hospital following surgery.
ABC News said it received a 23-page fax, apparently sent by Vester Flanagan under his professional name Bryce Williams later on Wednesday, in which he said his anger had been “building steadily” and that he had become “human powder keg” that was “waiting to go BOOM!!!!”
The writer says he suffered racism and homophobia at work, and expresses admiration for the teenagers who killed 13 people at Columbine High School in Colorado in 1999.
Vester Falanagan also said the attack in Charleston, South Carolina, in which nine black churchgoers were killed in June this year, was what “sent me over the top”.
Franklin County Sheriff Bill Overton said the ABC fax was being used by investigators, adding: “It’s obvious… this gentleman was disturbed in some way.”
Late on Wednesday, a representative for Vester Flanagan’s family issued a statement expressing their “deepest condolences to the families of Alison Parker and Adam Ward”.
“Our thoughts and prayers at this time are with the victim’s families and with WDBJ television station family,” the statement added.
President Barack Obama repeated his call for tougher gun laws after the attack.
“We’re willing to spend trillions of dollars to prevent terrorist activities, but we haven’t been willing so far at least to impose some common sense gun safety measures,” he said.