1. four years.
Four different years

2. 2001
Two planes flew into the twin towers in New York on September 11 2001. Also a
plane that crashed into the pentagon and another into a field.

3. 2007
The Virginia Tech massacre was a school shooting that took place on April 16,
2007, on the campus of Virginia Polytechnic Institute and State University in
Blacksburg, Virginia, United States. Seung-Hui Cho, a senior at Virginia Tech, shot
and killed 32 people and wounded 17 others in two separate attacks,
approximately two hours apart, before committing suicide

4. 2003
John Allen Muhammad, and one minor, Lee Boyd Malvo, was driving a blue
caprice while one of them layed in the trunk and killed 10 people and critically
injured 3 others.

5. 2013
• On September 16, 2013, Aaron Alexis, a lone gunman armed initially with a shotgun, fatally
shot twelve people and injured three others in a mass shooting at the headquarters of the
Naval Sea Systems Command (NAVSEA) inside the Washington Navy Yard in Southeast
Washington, D.C.[6][7][8] The attack began around 8:20 a.m. EDT in Building 197. Alexis was
killed by police around 9:20 a.m. EDT.