Their 16-year-old son was arrested and was being held as a juvenile.
Police did not speculate on a motive for the killings. But friends said the son liked being home-schooled but chafed against his parents' rules, which included a ban on drinking soda and monitored use of the Internet.
"His parents were really uptight on him," William Castillo said.
Another friend, Jonathan Marcum, said the teen was banned from watching R-rated movies and his parents subscribed to a service that removed adult material from PG-13 movies.
"He was very sheltered," Marcum told the Dallas Morning News. "If he broke the guidelines, he got punished."
The boy had four siblings -- one a recent college graduate, two in college and a sister in high school.
Edith Benitez, who lives across the street, said the boy had been a playmate of her children for years. She said she saw him mowing the lawn every week.
"It's still a shock to us," Benitez said. "He was a really good kid."