The United States has been waging a drone war across Africa and the Middle East for more than a decade. Sonia Kennebeck’s new PBS documentary "National Bird," executive produced by Errol Morris and Wim Wenders, tells the haunting stories of three American whistleblowers attempting to return to normal life after a stint in the highly controversial program.

Daniel, a political activist and former NSA signals intelligence analyst, is being investigated for espionage.

“He could be indicted any day, or years from now, for espionage, because the government suspects that he is a source of information about the drone program that the government doesn’t want out there," Daniel's attorney, Jesselyn Radack, explains in the film.

Heather, a military drone pilot cum full-time masseuse, says she saw the daily events unfold in "slow-motion."

"When you watch someone in those dying moments… it's so primitive," she says.

Specific deaths are etched in her memory, but what she finds most horrifying is her inability to recall them all.

"They don't report to us who we killed. Maybe we killed our objective, maybe we killed a guy who we thought was our objective," she explains. "I don't know how many people I've killed."

The bloodshed day in and day out left Heather feeling "empty."

"It's such an adrenaline rush," she admits. "You see someone die, because you said it was okay to kill them."

Then there's Lisa, a former technical sergeant on Drone Surveillance System, who speaks about another kind of death toll.

"When I first got into the military, I was thinking it was kind of a win-win; it was a force for good in the world," she recalls. "I thought I was going to be on the right side of history, and today I don't believe I was."

Lisa worked on a DGS, a distributed ground system.

"The system spans the globe," she explains. "It eats lots and lots of data… getting information [from] anywhere at any time, shooting people from anywhere at any time… There's a huge system… that can just suck up endless amounts of your life."

Because it's a secret program, Lisa can't reveal everything she knows.

"What I can tell you [as] one person who worked within this massive thing [is] it's frightening," she says.

National Bird airs on Independent Lens on Monday, May 1 on PBS.

Watch an exclusive clip from National Bird: