Update 8.27pm: 10 people were killed and 10 wounded in the shooting at a high school in Santa Fe, Texas governor Greg Abbott says.
The Texas student in custody over the school shooting played football and danced as part of a church group.
Two law enforcement officials have identified a person in custody in the Houston-area school shooting as 17-year-old Dimitrios Pagourtzis.
A woman who answered the phone at a number associated with the Pagourtzis family declined to speak.
"Please don't call us. Give us our time right now, thank you," she said.
Pagourtzis played on the school's junior varsity football team and was a member of a dance squad with a local Greek Orthodox church.
Father Stelios Sitaras, of Assumption of the Virgin Mary Greek Orthodox Church in Galveston, Texas, said he met Pagourtzis when the young man danced as part of an annual festival in October.
He said the Pagourtzis family are members of a nearby parish.
Mr Sitaras said he had never heard of the teenager being in any sort of trouble.
"He is a quiet boy," the priest said. "You would never think he would do anything like this."
A 16-year-old boy who considers Pagourtzis a friend said the suspect is interested in guns and war simulation video games, but has never about talked about killing.
Tristen Patterson is a junior at Sante Fe High School and said Pagourtzis did not show signs of being bullied but rarely talked about himself.
He said Pagourtzis would sometimes enter the classroom "acting a little bit down or sad. A little bit sluggish. ... But he never talked about why".
Update 7.10pm: The Associated Press have reported that police have identified one of the people in custody for the shooting.
They have been named as 17-year-old Dimitrios Pagourtzis.
Update 6.30pm: Harris County Sheriff Ed Gonzalez said he could not be precise about the number of deaths at Santa Fe High School.
Authorities said possible explosive devices have been found at the school and experts are in the process of rendering them safe.
There is no indication how many devices have been found. Police asked the public to "remain vigilant" and to call 911 if they see any suspicious items.
A school police officer is being treated in hospital, the sheriff said, but there was no immediate word on the extent of his injuries.
"We hope the worst is over, and I really can't say any more about that because it would be pure speculation," assistant principal Cris Richardson told reporters at the scene.
One student told Houston television station KTRK in a telephone interview that a gunman came into her first-period art class and started shooting. She said she saw one girl with blood on her leg as the class evacuated the room.
"We thought it was a fire drill at first but really, the teacher said, 'Start running'," the student told the television station.
She said she did not get a good look at the gunman because she was running away. She said students escaped through a door at the back of the classroom.
Update 4.55pm: Between eight and 10 people have been killed in a high school shooting in Texas, most of them students, according to police.
Officers said two people believed to be students have been detained after the incident at Santa Fe High School, about 30 miles south east of Houston.
Harris County Sheriff Ed Gonzalez said a police officer was injured but the extent of the officer’s injuries is unknown.
Vice president Mike Pence said he and President Donald Trump have been briefed on the shooting.
School shooting in Texas. Early reports not looking good. God bless all!— Donald J. Trump (@realDonaldTrump) May 18, 2018
He said the students, families, teachers and all those affected should know: “We’re with you. You’re in our prayers and I know you are in the prayers of the American people.”
Mr Trump added in a tweet: “Early reports not looking good. God bless all!”
First lady Melania Trump tweeted: “My heart goes out to Santa Fe and all of Texas today.”
- Press Association
Update 4pm: Police are reporting fatalities after a shooting at a Texas high school, according to local media.
Television station KHOU and the Houston Chronicle cited unnamed federal, county and police officials following the shooting at Santa Fe High School, about 30 miles south east of Houston.
The school district confirmed an unspecified number of injuries but is not immediately releasing further details.
Local media are reporting eight fatalities.
Assistant principal Cris Richardson said a suspect has been “arrested and secured”.
- Press Association
Update 2.37pm: Police are responding to a high school in Texas after a gunman was reported on campus.
The Santa Fe school district near Houston issued an alert saying Santa Fe High School had been placed on lockdown.
Galveston County Sheriff’s Major Douglas Hudson said units are responding to reports of shots fired. He had no immediate details on whether anyone had been hurt.
Santa Fe is a city of about 13,000 residents, 30 miles south east of Houston.
One student told Houston television station KTRK in a telephone interview that a gunman came into her first-period art class and started shooting.
The student said she saw one girl with blood on her leg as the class evacuated the room.
Authorities have not yet confirmed that report.
- Press Association