ROANOKE, United States (AFP) - The former television reporter who shot dead two journalists during a live US broadcast before killing himself warned he had been a "human powder keg... just waiting to go BOOM."

The gunman - Vester Lee Flanagan, 41, also known as Bryce Williams - posted chilling footage of Wednesday's shocking double murder online.

Reporter Alison Parker, 24, and cameraman Adam Ward, 27, were shot and killed at close range while conducting an on-air interview for WDBJ, a CBS affiliate in Roanoke, Virginia, about 385km south-west of Washington.

Friends, family and the community at large mourned the tragedy, which renewed calls for tougher gun laws in the United States. Flanagan was said to have bought his gun legally.

"It breaks my heart every time you read or hear about these kinds of incidents," President Barack Obama told an ABC affiliate in Philadelphia.

The killing also raised questions about how the Internet provided a brief but unfiltered window on a horrific crime.

WDBJ's morning newscast Thursday held a moment of silence 24 hours to the minute after Parker and Ward were killed.

"We will, over time, heal from this," said a grief-stricken morning anchor, Kimberly McBroom, holding hands with two colleagues.

Video of the shooting - apparently filmed by Flanagan himself - was posted on Twitter and Facebook. The footage was later removed.

Parker was interviewing Vicki Gardner, head of the Smith Mountain Lake Chamber of Commerce, at the lakeside Bridgewater Resort near Roanoke.

Several shots were heard, as well as screams, as Ward's camera fell to the floor, capturing a fuzzy and chilling glimpse of the gunman and his handgun.

Later, a video posted by Flanagan under the Twitter account #bryce-williams7, showed the shooter brandishing a weapon at Parker.

Both she and Ward apparently did not see the gunman.

Gardner, 62, was wounded in the back in the attack. She was reported in good condition Thursday after undergoing emergency surgery.

In New York, ABC News said it received a 23-page manifesto from a man identifying himself as Bryce Williams nearly two hours after the shooting.

In the rambling manifesto, Flanagan - an African American sacked in 2013 by WDBJ - said he was sent over the edge by the June mass shooting of black worshippers at a church in South Carolina.

He described himself as a "human powder keg... just waiting to go BOOM!!!!" Flanagan also complained in what he called a "Suicide Note for Friends and Family" of racial discrimination and bullying "for being a gay, black man." Jeffrey Marks, the station manager, said Flanagan was dismissed "after many incidents of his anger coming to the fore." Speaking on WDBJ Thursday, he described the manifesto as "nonsense."

Parker's fiance, Chris Hurst, the evening anchor at WDBJ, remembered her as "the love of my life" who was looking forward to preparing a major series on hospice care in Virginia.

"What great things she could have done," he said.

The cameraman's fiancee, Melissa Ott, the morning producer at WDBJ, meanwhile watched the shooting play out on air from the control room.

Ott was working her last day at WDBJ before moving on to another station in another city, and looking forward to a farewell party with her colleagues.

Parker's father, Andy Parker, made a plea for tougher gun laws after the murder of his daughter, who he described as "our bright, shining light." "We've got to do something about crazy people getting guns," he told Fox News.

US lawmakers have been hesitant to enact tougher limitation on access to guns, in part because they are loath to anger voters who fiercely defend their constitutional right to bear arms.