SANTA FE - At least eight people were killed when a student opened fire at his Texas high school on Friday, as President Donald Trump expressed "heartbreak" over the latest deadly school shooting in the United States.

The shooting took place as classes were beginning for the day at Santa Fe High School in the city of the same name, located about 30 miles (50 kilometers) southeast of Houston.

"There are multiple fatalities," Harris County Sheriff Ed Gonzalez told reporters. "There could be anywhere between eight to 10, the majority being students."

At least six people were hospitalized, including a police officer, Santa Fe school district police chief Walter Braun said.

Hospital officials said one victim was in critical condition.

Braun also said "explosive devices" had been found in the school and "surrounding areas" and warned people to exercise caution.

The male teen suspect in custody was not immediately identified. Media reports said he was 17 years old.

A second possible person of interest was being questioned, Gonzalez told reporters.

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.

"I shouldn't be going through this. It's my school. This is my daily life," said a weeping Santa Fe student, Dakota Shrader. 

"I feel scared to even go back. As soon as the alarms went off 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."

- Multiple shots -

Earlier this year, 17 students and staff were killed at a Florida high school -- a massacre that prompted survivors to launch a grassroots campaign against gun violence.

A somber Trump expressed "sadness and heartbreak" over the latest shooting spree.

"This has been going on too long in our country," said the president, 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 his administration 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.

"There was someone that walked in with a shotgun and started shooting, and this girl got shot in the leg," an eyewitness identified as Nikki told local television station KTRK.

The school district, which quickly imposed a lockdown, cleared the students from the campus, with television footage showing them leaving in a single file.

Some students were evacuated to a nearby auto shop, where parents were picking up their children, according to CBS affiliate KHOU-TV. 

Parent Richard Allen told KTRK he arrived at the school soon after the shooting and reported seeing a number of victims being taken away in ambulances.

"My son said someone went into the art room and started shooting a lot of the kids," Allen said. 

Student Paige Curry told local media that she heard gunshots and then a fire alarm, after which students were taken out by teachers to a nearby gas station.

"I saw some girl -- she had, you know, she got shot in the kneecap, I guess. So she had a bandage around it. She was limping and then the firemen came and got her," another student, Tyler Turner, told CBS.

A large contingent of police as well as agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives responded to the incident.

The Santa Fe shooting came three months after the shooting at Marjory Douglas Stoneman High School in Parkland, Florida, which sparked a student-led movement for tighter gun laws.

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

"We cannot let this continue to be the norm," tweeted Delaney Tarr, another student at Stoneman Douglas. "We cannot."