It didn't start out that way, but a new documentary tying various threads among far-right extremists and Oklahoma City bomber Timothy McVeigh serves as a history lesson and, indirectly, as a warning that something so horrible could happen again.

"Oklahoma City," directed by Barak Goodman, airs Tuesday night in PBS' "American Experience" series (9 p.m. EST) after its premiere last month at the Sundance Film Festival. Producer Mark Samels developed it as a means to "excavate" the story behind the bombing, Goodman said.

"This was hatched a couple years ago," Goodman said in an interview. "It was time to take a look at this worst case of domestic terrorism in American history ... and find the roots of the story."

The two-hour documentary unpacks separatist and white supremacy movements that dogged the country in the 1980s and early 1990s — detailing anti-government rhetoric that still echoes. McVeigh's involvement grows from selling anti-government bumper stickers in Texas to packing a Ryder truck with racing fuel and fertilizer and blowing it up in Oklahoma.