School shootings, mass killings are 'contagious,' study finds
Photos: Worst mass shootings in the United States
Parents wait for news after a shooting at Marjory Stoneman Douglas High School in Parkland, Florida, on Wednesday, February 14. At least 17 people were killed at the school, Broward County Sheriff Scott Israel said. The suspect, 19-year-old former student Nikolas Cruz, is in custody, the sheriff said. The sheriff said he was expelled for unspecified disciplinary reasons.
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Photos: Worst mass shootings in the United States
Investigators at the scene of a mass shooting at the First Baptist Church in Sutherland Springs, Texas, on Sunday, November 5, 2017. A man opened fire inside the small community church, killing at least 25 people and an unborn child. The gunman, 26-year-old Devin Patrick Kelley, was found dead in his vehicle. He was shot in the leg and torso by an armed citizen, and he had a self-inflicted gunshot to the head, authorities said.
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Photos: Worst mass shootings in the United States
A couple huddles after shots rang out at a country music festival on the Las Vegas Strip on Sunday, October 1, 2017. At least 58 people were killed and almost 500 were injured when a gunman opened fire on the crowd. Police said the gunman, 64-year-old Stephen Paddock, fired from the Mandalay Bay hotel, several hundred feet southwest of the concert grounds. He was found dead in his hotel room, and authorities believe he killed himself and that he acted alone. It is the deadliest mass shooting in modern US history.
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Photos: Worst mass shootings in the United States
Police direct family members away from the scene of a shooting at the Pulse nightclub in Orlando in June 2016. Omar Mateen, 29, opened fire inside the club, killing at least 49 people and injuring more than 50. Police fatally shot Mateen during an operation to free hostages that officials say he was holding at the club.
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Photos: Worst mass shootings in the United States
In December 2015, two shooters killed 14 people and injured 21 at the Inland Regional Center in San Bernardino, California, where employees with the county health department were attending a holiday event. The shooters, Syed Rizwan Farook and his wife Tashfeen Malik, were later killed in a shootout with authorities. The pair were found to be radicalized extremists who planned the shootings as a terror attack, investigators said.
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Photos: Worst mass shootings in the United States
Police search students outside Umpqua Community College after a deadly shooting at the school in Roseburg, Oregon, in October 2015. Nine people were killed and at least nine were injured, police said. The gunman, Chris Harper-Mercer, committed suicide after exchanging gunfire with officers, a sheriff said.
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Photos: Worst mass shootings in the United States
A man kneels across the street f