THE suspect accused of attacking a Texas high school exchanged "a lot of firepower" with authorities before he surrendered, a top county official says, a day after the shooting in an art classroom that killed 10 people.

In addition to a shotgun and a handgun, Dimitrios Pagourtzis, 17, also had several kinds of homemade explosive devices, but they were not capable of detonating, said Mark Henry, the Galveston County judge and chief administrator.

Investigators found a group of carbon dioxide canisters taped together, and a pressure cooker with an alarm clock and nails inside. But the canisters had no detonation device, and the pressure cooker had no explosive material, Henry said.

Authorities offered no motive, but they said in a probable-cause affidavit that the suspect had admitted to carrying out the shooting.

The gunman told police that when he opened fire, he avoided shooting students he liked "so he could have his story told", the affidavit said.

Pagourtzis, who appeared to have no prior arrests or confrontations with law enforcement, made his initial court appearance on murder charges on Friday by video link from the Galveston County Jail. A judge denied bond and took his application for a court-appointed attorney.

The shooting unfolded in Santa Fe, a city of about 13,000 people roughly 48 kilometres southeast of Houston. Among the dead were nine students, including a foreign exchange student from Pakistan, and a substitute teacher.

The wounded included a school police officer who was the first to confront the suspect and got shot in the arm. Hospitals reported treating a total of 14 people for injuries related to the shooting. Two were listed in critical condition on Saturday at the University of Texas Medical Branch, the hospital tweeted.

The gunman yelled "Surprise" before he started shooting, according to Texas Representative Michael McCaul, the chairman of the House Homeland Security Committee.

Pagourtzis played on the junior varsity football team and was a member of a dance squad with a local Greek Orthodox church. Acquaintances described him as quiet and unassuming, an avid video game player who routinely wore a black trench coat and black boots to class.

The suspect obtained the guns from his father, who owned them legally, Governor Greg Abbott said. It was not clear whether the father knew his son had taken them.