DENTON, Texas—Opening arguments started Tuesday in the capital murder trial of a 21-year-old man accused of killing a customer during a botched robbery at a smoke shop in Dallas.

Arsenio Desmond Petty was arrested in January for the Dec. 30 shooting death of 18-year-old Rabeca Rae Kemp at the Gas Pipe. The store sells rolling papers, water pipes and other items.

Three other people were injured.

Initially, police said robbery did not appear to be the motive, in part because the gunman appeared to have fired without making any demands. But further investigation led investigators to believe it was a “botched” robbery.

A second man, Maurice Antione Hall, of Allen, was also accused of capital murder in the shooting.