Investigators said Cruz ordered an Uber ride to the school, where he opened fire and roamed the hallways for several minutes.
"Here's the plan: I'm going to go take Uber in the afternoon before 2:40," he said. "From there, I'll go to the school campus, walk up the stairs, load my bags, take my AR and shoot people."
Prosecutors didn't say when the clips were recorded.
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"I live a lone life, live in seclusion and solitude. I hate everyone and everything," he said. "The power of my AR – you will all know who I am. I had enough of being told what to do and when to do (it)."
Cruz said he expected to become well-known for his plans.
"When you see me on the news, you'll all know who I am," he said.
Investigators said Cruz confessed to the crimes. He could face the death penalty if convicted.