Vester Flanagan, who had been dismissed by the same station, WDBJ7, killed himself after a police chase.
In the fax, a man said to be 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 and cameraman Adam Ward. Station manager Jeff Marks said: "I cannot tell you how much they were loved."
He said Flanagan was an "unhappy man" who had to be escorted from the WDBJ7 building after being dismissed in 2013.
Ms Parker was conducting a live interview with a guest on tourism early on Wednesday in the town of Moneta when the incident occurred.
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, the gunman posted footage online of himself opening fire at close range, although this was later removed.
The interviewee, 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 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.
He 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 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.
However Republican presidential hopefuls Jim Gilmore and Ben Carson have warned against any rush to introduce tougher gun controls.
"We're not going to let this madman take away the lawful rights of Americans to keep and bear arms. We're going to stand up for the second amendment," Jim Gilmore, a former governor of Virginia, told CNN.
- Aged 24, was a reporter for WDBJ7's Mornin' show
- Grew up in Martinsville, Virginia and edited her university newspaper
- Joined WDBJ7 after internship and said she grew up watching the station
- Was dating station anchor Chris Hurst
- Aged 27, was a cameraman for WDBJ7
- Went to school in Salem, Virginia, and graduated from Virginia Tech
- Described by his employer as committed and a "fine photojournalist"
- Was engaged to station producer Melissa Ott
The Twitter and Facebook accounts of Bryce Williams, Flanagan's on-air name, have been suspended
The live footage apparently captured the gunman