STEM School Highlands Ranch shooting

On May 7, 2019, a school shooting occurred at STEM School Highlands Ranch, a K-12 charter school of approximately 1,850 students in Douglas County, Colorado, in the Denver suburb of Highlands Ranch. At the time, the school had no assigned police officer and relied on private security. An anonymous parent had reportedly raised concerns about bullying and violence at the school in December 2018, prompting a district official's letter urging investigation; the school later filed a defamation lawsuit against that parent.

At 1:53 p.m., 18-year-old Devon Erickson and 16-year-old Alec McKinney entered the school carrying handguns and other weapons concealed in a guitar case and backpack. They opened fire in classroom 107 from two doors on opposite sides of the room while students were watching a film. Erickson drew a handgun and ordered students not to move; student Kendrick Castillo lunged at him and was fatally shot in the chest. Two other students, Joshua Jones and Brendan Bialy, also confronted Erickson, subduing and disarming him; Jones was shot and injured. Simultaneously, McKinney fired a revolver, emptying it and striking four students, before being tackled by other students and later surrendering to a security guard after struggling with the safety mechanism on another handgun. Officials said McKinney later told investigators he had intended to commit suicide after the attack. Police arrived within two minutes of the first 911 call, aided by a direct radio link between the school and the sheriff's dispatch center. One instance of friendly fire occurred when a private security guard reacted to a Douglas County Sheriff's deputy's weapon. Investigators recovered three handguns and a rifle, and found graffiti reading "Fuck society," "666," and a pentagram on a car at Erickson's home.

Kendrick Castillo, an active member of the school's robotics team, was the only student killed; the youngest of the eight injured victims was reported to be 15. All victims were students; there were no staff casualties. The last injured student was released from the hospital on May 12, 2019.

Both suspects were students at the school and were not previously known to law enforcement. Erickson was booked on 30 counts, including first-degree murder and 29 counts of attempted first-degree murder; both he and McKinney were later charged with 48 counts, including first-degree murder, arson, and burglary. McKinney, tried as an adult after a judge denied a motion to move his case to juvenile court, pleaded guilty in February 2020 to 17 charges and was sentenced on May 18, 2020, to life with the possibility of parole after 40 years, plus 38 years on other charges. Prosecutors announced in March 2020 that they would not seek the death penalty against Erickson. His trial began May 31, 2021; McKinney testified for the prosecution. On June 15, 2021, Erickson was convicted on 46 counts, including first-degree murder and conspiracy to commit first-degree murder, and on September 17, 2021, was sentenced to life without parole plus 1,282 years.

The shooting prompted statements from President Donald Trump, White House officials, and members of Congress, as well as community vigils, a memorial fund, and later commemorations, including renaming a local street "Kendrick Castillo Way" and a 2025 petition to open a Catholic sainthood cause for Castillo.