A 17-year-old student confessed to opening fire at his Texas high school on Friday, killing 10 people, and told investigators that he had spared certain students "so he could have his story told," the authorities said.

A Galveston County Sheriff's Office investigator wrote in an affidavit that Dimitrios Pagourtzis had waived his right to remain silent and had given "a statement admitting to shooting multiple people" at Santa Fe High School. The investigator, identified only as J. Roy, also wrote that Pagourtzis had said that "he did not shoot students he did like so he could have his story told."

Pagourtzis' apparently well-planned assault in this rural community of about 13,000 people was the deadliest school shooting since February, when 17 people were killed in Parkland, Florida. But investigators said that Pagourtzis had given no overt indications that he was planning a mass shooting.

"Unlike Parkland, unlike Sutherland Springs, there were not those types of warning signs," Governor Greg Abbott of Texas said, referring to the Florida school attack and a siege at a church last November. "We have what are often categorised as red-flag warnings, and here, the red-flag warnings were either non-existent or very imperceptible."

In the aftermath of Friday's attack, though, Abbott said that investigators had recovered information from Pagourtzis' computer and cellphone that suggested that he wanted to carry out the shooting and to kill himself afterward. Instead, Pagourtzis surrendered and was being held on Saturday without bond on charges of capital murder and aggravated assault of a public servant.