Dimitrios Pagourtzis, 17, kept his head bowed during a hearing to determine if there was probable cause to hold him with no bond.
"His demeanor was just standing there. He really didn't say anything. I asked him if he had any questions he didn't have any questions," said Galveston County County Judge Mark Henry who presided over the hearing.
Pagourtzis, described by other students as a quiet loner, admitted to police that he opened fire on fellow students.
A court document states the high school junior told police he did not shoot students he did like so he could have his story told.
"He apparently was cooperating with police answering questions, agreed to be interviewed, waived his rights to remain silent," Henry said.
Earlier, police described an individual as a person of interest, but now they say Pagourtzis acted alone.
"The details I got lead me to believe that's a correct assessment," Henry said. "I don't see any real connection from what they're telling me and I don't expect they're going to end up being an accomplice."
The teenager posted photos of a trench coat and a shirt that reads "born to kill" on social media. Police say he wore those items when he used his father's shotgun and .38 revolver to kill eight students and two teachers.
His explosives, authorities say, were not functional.
"They were CO2 canisters wrapped with duct tape, no way to detonate, and a pressure cooker with an alarm clock and some nails. No explosive device," the county judge said.