Two people have been killed and 17 injured in a high school shooting in the US state of Kentucky.
A 15-year-old girl died at the scene at Marshall County High School in the town of Benton and a 15-year-old boy died in hospital, said Kentucky's governor.
The unidentified attacker was a 15-year-old male student, who was arrested and is in custody.
State police said he was detained about 15 minutes after opening fire about 8am local time on Tuesday.
The lone attacker was a student at the school who opened fire in a common area before classes started, according to local media.
He will be charged with murder and attempted murder, said officials.
"To walk in, the backpacks laying around, the phones laying around, going off, it's indescribable," Marshall County Attorney Jeffery Edwards told the Louisville Courier-Journal.
"I've been doing this for 25 years. It's not like anything I've experienced in my life."
Authorities offered no information on the motive behind the shooting. There was no immediate indication how well the student, who officials said faces multiple charges of murder and attempted murder, knew the victims.
Kentucky Governor Matt Bevin said there was "no good answer".
"There's 1,000 hypotheses we're not going to go into."
The bloodshed at the school, where nearly 1,150 students are enrolled, was the latest outbreak of gun violence that has become a regular occurrence at schools and college campuses across the United States over the past several years.
Tuesday's carnage occurred 52 km from Heath High School in West Paducah, Kentucky, where in 1997 a 14-year-old boy opened fire on a group of students, killing three.
Five of the 12 students with gunshot wounds were taken by helicopter to the Vanderbilt University Medical Center's trauma centre in Nashville, a hospital spokeswoman said.
Another five students suffered non-gunshot injuries.
Agents from the US Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) and the FBI were taking part in the investigation, officials said at the news conference.
During the news conference at the county Board of Education, Mr Bevin paused to collect himself as his voice choked with emotion, asking members of the news media to exercise restraint in dealing with the families of victims.
"This is a wound that is going take a long time to heal," he said.
"And for some in this community it will never fully heal."