The witness recounted hearing the rifle discharge, prompting him to turn around and discover the victim on the ground.
The second man was transported to a hospital with injuries deemed to be life-threatening.
Officials are collaborating with security personnel at the Alamo Draft House to collect potential suspect details.
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The 20-year-old man also succumbed to his injuries.
Two firearms were recovered by authorities at the scene.
No injuries have been reported.
Julian Howard, a 17-year-old student, has been arrested and formally charged with murder.
Detectives are still working to determine a motive, but it appears the suspect and victim knew each other.
Walker has been transported to Dallas County Jail.