If threw these nails out into the crowd, extremely deadly shrapnel.

Once the FBI knows what materials were used to build the bomb and how it was concealed, agents make a public appeal for information about the bomber.

Today in showing you this backpack of this reconstructed bomb here, we believe we've given every possible opportunity to recount to the public to refresh their memories and perhaps can follow additional information.

But Sachtleben reveals another secret for rebuilding bombs.

Actually go to a bombing range and use, and sometimes, thousands of pounds of explosives inside of a vehicle that we think similar to the one used, detonate it. Go out, measure the crater, measure where the pieces were, were found, collect the evidence that we find there and compare it to what we had at the bombing scene.

There is also another reason for these dramatic recreations. These images give jurors an unforgettable look at the devastation a bomb can cause.

Sometimes, we'll actually go to our explosive range and reconstruct the device, detonate it, film that, so we can go into court and show that to the judge and jury.

Eric Rudolph, the man accused of the bombings of both the Olympics and the abortion clinic in Atlanta has yet to face a jury, he remains at large. But thanks to the Explosives Unit, McVeigh and Kaczynski will never again pose a threat to society. However, the FBI's ultimate goal is to catch terrorists before they strike.

Bridgeport, Texas, the residents of this sleepy farm town outside Dallas have no idea that their lives are in danger. That's because the people caught on this surveillance video are planning a terrorist attack.

The suspected bombers are members of the Ku Klux Klan. They are scheming to destroy the Mitchell Energy refinery as a diversion for another crime, an armored car heist which would fund the war they are planning against the US government.

But these terrorists are unaware that the FBI is watching their every move, that's because one of their own is secretly working undercover for the FBI as an informant.