Tomorrow night the PBS program American Experience airs a documentary on what remains the worst act of domestic terrorism in American history.

On April 19, 1995, Timothy McVeigh parked a rental truck loaded with a five-ton fertilizer bomb in front of the Murrah Federal Building in Oklahoma City. It was the kind of bomb that leveled the Marine barracks in Beirut, Lebanon in 1983, killing 241 US service members. It was the kind of bomb that the IRA used to turn the heart of London’s financial district to rubble in 1993.

By the time all the bodies were counted after that April morning in 1995, 168 people lay dead, including 19 children, 15 of whom had been in the building’s daycare center. Another 675 were injured in the blast.

If you’ve followed this blog at all, you know that a regular theme of mine has been trying to highlight the true nature of the terrorist threat that faces the United States. I won’t link to all the earlier posts, but you can get to several of them from here.

The bottom line remains the same, and PBS’ “Oklahoma City” reminds us of the fact: The biggest terrorist threat to America comes not from immigrants or refugees but from other Americans.

You can watch the first chapter of the film below, and tune in to your local public television station tomorrow night for the full documentary.