What's more, the other two firearms found at the scene -- a Glock handgun and a Sig Sauer handgun -- were also not affected by the ban.
Was the Ban Effective at Reducing Gun Violence In General?
That is unclear. According to a 2004 study from the University of Pennsylvania, the number of people killed in mass shootings did go down generally during the years that the ban was in effect. The exception was 1999, the year that the shooting at Columbine High School happened.
The number of mass shootings per year has doubled since the ban expired, but the researchers say it's difficult to discern whether there was a cause-and-effect relationship.
The study found that gun crimes involving assault weapons declined by as much as 72 percent in the localities examined after the ban went into effect. However, the authors note that these types of weapons were only used in 2 to 8 percent of the gun crimes committed prior to the ban, so the larger impact on gun violence was minimal.
"We cannot clearly credit the ban with any of the nation's recent drop in gun violence," the study concluded. "And, indeed, there has been no discernible reduction in the lethality and injuriousness of gun violence."
Gun control proponents argued the assault weapons ban was too narrow, and its language was too lenient. There were hundreds of assault weapons unaffected by the law, they said, and many loopholes that allowed for the continued manufacturing of models similar to those banned.