A 17-year-old armed with a shotgun and pistol has opened fire in a Texas high school, killing nine fellow students and a teacher in an attack similar to the massacre at a Florida high school in February.
Students say the gunman, identified by law enforcement as Dimitrios Pagourtzis, first opened fire in an art class at Santa Fe High School shortly before 8am (11pm AEST) on Friday.
Students and staff fled after seeing classmates wounded and a fire alarm triggered a full evacuation.
Ten people were also wounded in the attack, according to Texas Governor Greg Abbott, who said the suspect left behind explosive devices.
He said investigators had seen a T-shirt on the suspect's Facebook page that read "Born to Kill" and authorities were examining his journal.
However, there were no outward signs he had been planning an attack.
"Here, the red flag warnings were either non-existent or very imperceptible," Abbott told a news conference.
Classmates described the Santa Fe suspect as a quiet student who kept to himself and played on the football team.
They also said he wore a trench coat to school on the day of shooting when temperatures topped 32C.
It was the latest in a long series of deadly shootings at US schools.
Seventeen teens and educators were shot dead at a Parkland, Florida, high school on February 14, a massacre that stirred the nation's long-running debate over gun ownership.
It was the second mass shooting in Texas in less than a year, following a massacre at a rural church in November, when a man armed with an assault rifle shot dead 26 people.
The Galveston County Sheriff's Office said Pagourtzis was being charged with capital murder and more charges could follow.
Abbott said the suspect in the fourth-deadliest mass shooting at a US public school had used firearms taken from his father.
"Not only did he want to commit the shooting but he wanted to commit suicide after the shooting," Abbott said, citing a police review of the suspect's journals.
Two other people were in custody, he said.
Explosive devices were also found at the school, about 50km southeast of Houston, and off campus, Harris County Sheriff Ed Gonzalez tweeted.
Police searched two homes and a vehicle linked to the suspect, where they found multiple homemade explosive devices.
US President Donald Trump called the latest school massacre "absolutely horrific".
"My administration is determined to do everything in our power to protect our students, secure our schools and to keep weapons out of the hands of those who pose a threat to themselves and to others," Trump said at the White House.
Days after the Parkland shooting, Trump said elected officials should be ready to "fight" the powerful National Rifle Association lobby group.
However, early this month he embraced that group, telling its annual meeting in Dallas "your Second Amendment rights are under siege".
No major federal gun controls have been imposed since Parkland.