The gunman, arrested on murder charges, was identified as Dimitrios Pagourtzis, a 17-year-old junior at Santa Fe High School. He is being held on capital murder charges, meaning he could face the death penalty.

Governor Greg Abbott said 10 people died and another 10 were wounded in "one of the most heinous attacks that we've ever seen in the history of Texas schools."

"Nothing can prepare a parent for the loss of a child," Abbott told reporters in Santa Fe, located about 30 miles (50 kilometers) southeast of Houston. 

Abbott said the gunman apparently carried out the attack with a shotgun and a .38 revolver that were legally owned by his father.

The Houston Chronicle said Pagourtzis also threw pipe bombs into a classroom at the start of the school day.

But with gun reform a divisive issue in the United States, Abbott focused on the need to better address mental health concerns and to arm schools, rather than call for limits on the proliferation of weapons.

At dusk, hundreds of people, including many children, turned out for a candlelight vigil as the community sought to cope with the tragedy. People prayed and sang "Amazing Grace," tears streaming down some faces. There were hugs, and dogs providing emotional support.

Abbott said searches were being conducted at two residences and "explosive devices" had been found, including a "CO2 device" and a Molotov cocktail.

He said journal entries by the gunman suggested he wanted to commit suicide but "he gave himself up."

Abbott said there were no "warning signs" about the suspect, who had no criminal history, although he did post a picture on his Facebook page of a T-shirt with the words "Born to Kill" on it.

Law enforcement authorities were questioning two "people of interest," the governor said. One may have "certain information," he said, and the other had some "suspicious reactions."

Harris County Sheriff Ed Gonzalez said most victims were students.

"Officers inside encountered a bloody mess in the school," a law enforcement official told the Houston Chronicle.

Area hospitals said two victims were in critical condition. One of the wounded was a police officer, John Barnes, who engaged the gunman and was shot in the elbow, officials said. 

The shooting was the latest in what has become an all-too-familiar situation in American schools, where gun violence has become a part of everyday life, part of a broader toll taken by firearms which are responsible for more than 30,000 gun-related deaths annually in the US.

"My friend got shot in the art hall," weeping student Dakota Shrader told reporters.

"I shouldn't be going through this," Shrader said as her mother comforted her. "It's my school. This is my daily life. I feel scared to even go back."

Shrader said fire alarms went off "and everybody just started running outside.

"And the next thing you know everybody looks and you hear 'boom, boom, boom' and I just ran as fast as I could to the nearest forest so I could hide and I called my mom."

Earlier this year, 17 people were killed at Marjory Douglas Stoneman High School in Parkland, Florida, -- a massacre that prompted survivors to launch a grassroots campaign against gun violence that led to the largest gun reform rallies in nearly two decades.

Friday's was the second mass shooting in Texas in six months.

Twenty-six people were killed in a Texas church in November 2017 by a 26-year-old Air Force veteran.

Abbott, the Texas governor, said he would convene roundtable discussions to look into ways of preventing such mass shootings in the future.

President Donald Trump expressed "sadness and heartbreak" over the shooting and ordered US flags to fly at half-staff for the next few days.

"This has been going on too long in our country," Trump said, speaking at an event on prison reform at the White House. "We're with you in this tragic hour."

Trump, who has previously shied away from gun control and proposed arming teachers, said he was "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."

The shooting erupted before 8:00 am, as the first classes of the day were starting.

"It was pretty quiet for a few seconds, and then we just hear someone shoot a gun, and just a 'pow-pow-pow,'" student Hunter Mead, 14, told AFP.

"And everyone just kind of freaked out. You know, everyone has the instinct, 'Just go, get out of here.'"

The Santa Fe shooting came three months after the Florida massacre led to mass protests and their call of "Never again!".

"We are fighting for you," tweeted David Hogg, a Stoneman Douglas student who has emerged as a leader of the gun control campaign.

Another Stoneman Douglas student, Delaney Tarr, added: "We cannot let this continue to be the norm. We cannot."

Former Democratic presidential candidate Hillary Clinton urged lawmakers to take action.

"Every day that we fail to act on gun violence, we are failing our children," Clinton said.