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A pair of bombs went off near the finish line of the packed Boston Marathon on April 15, 2013, leaving three people dead. More than 200 people were wounded. One suspect, Dzhokhar Tsarnaev, has been charged with 30 federal counts related to the attacks. His brother, Tamerlan, was killed during pursuit by police. David L. Ryan/The Boston Globe via Getty Images
In this handout photo provided by the U.S. Army, police take cover as Nidal Hasan opens fire in Fort Hood, Texas, on November 5, 2009. The Army psychiatrist was convicted of killing 13 people. Andrew Evans/U.S. Army via Getty Images
A man stands near the rubble, asking if anyone needs help, after the collapse of one of the World Trade Center towers on September 11, 2011. In what was the worst terrorist attack in U.S. history, 2,753 people were killed when two hijacked planes were intentionally crashed in the north and south towers of the New York buildings. Two other planes were also hijacked: One crashed into the Pentagon in Washington, and one crashed at a field near Shanksville, Pennsylvania. DOUG KANTER/AFP/Getty Images
Atlanta was the excited and elated host of the 1996 Summer Olympics when a bomb went off at Centennial Olympic Park on July 27. Two people were killed and 111 were injured by the blast. Eric Rudolph was convicted of placing the 40-pound bomb, which was filled with nails and screws. DIMITRI MESSINIS/AFP/Getty Images
The north side of the Alfred P. Murrah Federal Building in Oklahoma City shows the devastation caused by a fuel-and-fertilizer truck bomb on April 19, 1995. At the time, it was the worst terror attack on U.S. soil, killing 168 people. Timothy McVeigh and Terry Nichols were convicted of the attack. Both were former U.S. Army soldiers associated with the militant Patriot Movement. AFP/Getty Images/file
A police photographer helps document the bombing of the underground parking garage at the World Trade Center. That bombing killed six people on February 26, 1993. Six suspects were convicted of participating in the bombing. The seventh suspect, Abdul Rahman Yasin, is still at large. Ramzi Yousef directed the organization and execution of the bombing. He said he did it to avenge the sufferings Palestinian people had endured at the hands of U.S.-aided Israel. MARK D.PHILLIPS/AFP/Getty Images
Three people were killed and 23 others were wounded after a string of mail bombings carried out by Ted Kaczynski, aka "The Unabomber," from 1978 to 1995. Here, FBI agents guard the entrance to Kaczynski's property in Lincoln, Montana, on April 5, 1996. In May 1998, Kaczynski received eight life sentences for his crimes. JOHN RUTHROFF/AFP/Getty Images
