At a news conference, Kentucky State Police Commissioner Rick Sanders said the boy walked into Marshall High School in Benton, Kentucky, shortly before 8 a.m. and began shooting.
Gov. Matt Bevin said a 15-year-old girl was pronounced dead at the school and a 15-year-old boy succumbed to his injuries at a hospital. Twelve other people were shot, and five people were treated for other types of injuries sustained during the incident.
The teen shooting suspect, a student, faces murder and attempted murder charges, Bevin said.
"This is a wound that is going to take a long time to heal and for some in this community, it will never heal," Bevin said.
Jeff Edwards, the county attorney in Marshall County, toured the school after the shooting, and described evidence of the chaos that unfolded as students arrived at school.
"To walk in, the backpacks laying around, the phones laying around, going off . . . it's indescribable," Edwards told the Courier Journal in Louisville. "I've been doing this for 25 years. It's not like anything I've experienced in my life."
Five of those injured were taken to Vanderbilt University Medical Center in Nashville, Tennessee. Doctors there said they treated five boys between the ages of 15 and 18 with a range of injuries. One teen had been shot in the head, a second in the arm and a third in the chest and abdomen.
Kentucky State Police Lt. Michael Webb said the FBI and Bureau of Alcohol, Tobacco, Firearms and Explosives are assisting police as they process the scene at the high school. Authorities plan to search the shooting suspect's family home as they investigate his motives.
Sanders said students had recently been trained in how to respond to active shooters in their school hallways.
Authorities declined to release the names of victims or the suspect.
White House Press Secretary Sarah Huckabee Sanders said President Trump had been briefed on the shooting and that he offered "thoughts and prayers" for the victims and families.
Sen. Mitch McConnell, R-Ky., spoke Tuesday on the Senate floor, offering condolences to the community.
"Our hearts are with the entire community in Marshall County and our gratitude is with the first responders who rushed into harm's way," McConnell said.
Marshall County High School is in Benton, about 25 miles southeast of West Paducah, Kentucky, where a high school freshman opened fire on a student prayer group, killing three, in 1997.
Marshall High had 1,300 students in the 2014-2015 school year, according to the National Center for Education Statistics.
News of the shooting in Kentucky came just one day after an incident in Texas that left one teen injured. Early Monday morning, police responded to reports of an active shooter at Italy High School, in a town about 50 miles south of Dallas. A 16-year-old opened fire in the cafeteria, police said, wounding a 15-year-old girl who was airlifted to a Dallas hospital. Afterward, authorities took the boy into custody.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)