Police in the U.S. city of Boulder, Colorado Tuesday said a 21-year-old suspect is in custody and has been charged with 10 counts of murder in connection with a shooting at a Boulder grocery store Monday.Less than a week after the horrific murders of eight people in Georgia, another American city has been scarred by gun violence. Tune in as I deliver remarks. King Soopers, BoulderA law enforcement official briefed on the shooting told The Associated Press news agency that the gunman used an AR-15 rifle, a lightweight semi-automatic firearm. An official told AP they were working fast to trace the gun. The official was not authorized to speak publicly and spoke to AP on condition of anonymity.Police said the suspect made several statements to police when he was taken into custody but they did not release details of those statements. They said a motive for the shooting was unclear.Colorado has been the sight of several mass shootings in recent times, including a 2012 attack at a movie theater in Aurora and the 1999 attack at a school in Columbine.Monday’s attack comes a week after the killing of eight people at three Atlanta-area massage businesses. It was at least the seventh mass killing this year in the United States.A History of Mass ShootingsWho commits public mass shootings? What motivates them to kill? With the help of a landmark database, VOA examines the social, psychological, emotional and environmental factors that contributed to these rare crimes.