Denton County sheriff's deputies believe that in December, Dyson waited out his girlfriend, Vanessa Garces, 21, at her home near Copper Canyon.
They said he rammed her car with his pickup and opened fire, killing her. Dyson was taken to jail after a standoff with a SWAT team.
He was awaiting trial when, early Tuesday, he appeared to be having seizure. Deputies said they took him to the hospital, where he died.
Staff Writer Bill Miller reports.