Back at the explosives lab, examiners use a high-tech instrument called a Raman spectrometer, a device that determines if a package contains explosive material.

With the Raman, what we are doing is that we are using, a, a light source to go into the container and actually get a spectrum off of the substance inside the container. There we're actually able to examine a suspected explosive substance without even having to open the container. And that's great for in the laboratory when you don't wanna put yourself at risk any further than you have to.

At the federal building in Oklahoma City, no secondary devices are found. But while the nation is horrified at the carnage. The Explosives Unit is quickly searching the wreckage for evidence to be used in a test called: residue analysis. This helps the FBI determine what chemicals fuel the blast.

First thing that happens is... is that the part of our unit that does the residue analysis takes the evidence, extracts anything that they can find from there that would help them identify the explosives.

Within hours, residue analysis pinpoints a key piece of evidence.

There we were fortunate that we actually did find a piece of debris that had some of the ammonium nitrate prills that were still embedded in it. And from those prills, we were able to conclude that it was an ammonium nitrate based explosive.

From these findings the FBI concludes that a fertilizer bomb was used in the most deadly terrorist attack on US soil. Now the search for a suspect begins. The challenge for the Explosives Unit is to determine how the massive bomb was brought to the scene.

If it's a very large bombing, it's probably gonna be a vehicle. So we wanna try to identify that vehicle as quickly as we can. And here you can see some pieces that we did in a test with a vehicle. And you can just see the extent to which these are mangled by the explosion.

Within a short time, investigators located the axle of a truck that was blown nearly 600 feet away from the federal building.

From that axle, the bomb technicians who were at the scene were able to very quickly identify it as having come from the bomb vehicle, and then they were able to give that information to other investigators who tracked it back to the rental facility.

In less than a day, suspected bomber Timothy McVeigh is taken into custody. But examiners investigating the murder of the New Jersey ad executive are not so lucky. With no sign of a suspect, they focus their attention on the bomb itself and begin a process known as device analysis.

When we're doing an analysis of a device, what we were trying to do is simply to identify the components found there, and trying to see if we can figure out how they were put together, how they worked, or in some cases, how they didn't work?

Raman spectrometer: a device to analyze the intensity of Raman scattering of monochromatic light (单色光) as a function of frequency of the scattered light; the information obtained is useful for determining molecular structure