Click on the audio player above to hear this interview.

When the San Benardino shooters, Syed Rizwan Farook and his wife Tashfeen Malik, were shot and killed by police last week, they were found to have two AR-15 assault rifles and 2,500 rounds of rifle ammunition on them.

While that may sound alarming, Farook and Malik have something in common with millions of other Americans. About 8 million AR-15s have been sold around the country—it's the most popular gun in America.

Adam Lanza brought a shotgun, two handguns, and a Bushmaster semiautomatic rifle to Sandy Hook Elementary School. He also brought 24 different magazines and more than 500 rounds of ammunition. He left two bolt-action rifles and a pistol at home that day.

James Holmes brought a semiautomatic variation of the M-16 rifle, a pump-action gauge shotgun, and a .40-caliber semiautomatic pistol to a Colorado movie theater in 2012. In the months leading up to the shootings he also purchased 3,000 rounds of handgun ammunition, 3,000 rounds for a semiautomatic rifle, and 350 shotgun shells, all over the Internet.

Though purchasing ammunition by the thousands sounds quite large, they are often sold in 1,000 round boxes, and buying more at once often means a cheaper sale price.

Are these mass shooters just participating in American consumerism? And why didn't anyone notice when their gun collection transitioned from that of a hobbyist to a deadly arsenal? For answers, we turn to Michael Sullivan, former director of the Bureau of Alcohol, Tobacco, Firearms and Explosives from 2006 to 2009. 

What you'll learn from this segment:

• What federal law says about stockpiling weapons and ammunition.
• How people amass arsenals.
• Whether these individuals are monitored by law enforcement.