Officers said when they arrived they found Oday Qadous dead inside the Valero convenience store.
"He was carelessly gunned down by these horrible individuals who didn't care anything about anyone else's life or livelihood but their own," Houston Police Department Lt. W.L. Miller said.
Police said three suspects opened fire on the clerk within minutes of walking into the store.
Surveillance photos from inside the convenience store show one man wearing what looks like an alien mask while the other two wore black masks.
Police said the three suspects took off in a dark-colored Nissan. They believe the men didn't steal anything.
Officers said Qadous, a 29-year-old Palestinian man, had recently immigrated to the United States.
"This is horrible. This man, 29 years old, came to this country for a better life, to support his family, and he gets gunned down for it ... it's not right. These guys need to be caught," Lt. Meeler with Houston Police Department Homicide said.
Many of Qadous' friends showed up at the gas station after the shooting in disbelief.
"His mom, of course, was crying and everyone was freaking out. They are crying and are very, very sad. It is just very bad news," Quadous' friend Mohammad Issa said.