A Texas woman, who was held at gunpoint and killed along with her husband and six children, had sent a text message to her mother before she died.
Valerie Jackson wrote that she was being held hostage, a Texas prosecutor said, prompting her mother to call authorities.
The suspect David Conley, who had previously dated Jackson, appeared in court for the first time on Monday.
The eight victims were found dead on Saturday in their Texas home.
When officers arrived at the Houston-area home on Saturday, police said Mr Conley opened fired and barricaded himself inside.
Mr Conley surrendered about an hour later, after talks with police negotiators, and was charged with capital murder.
In addition to Valerie Jackson, the victims were identified as Dewayne Jackson, 50, and children Nathaniel, 13, Dewayne, 10, Honesty, 11, Caleb, 9, Trinity, 7, and Jonah, 6.
Police said some of the victims were found handcuffed inside their bedrooms.
Nathaniel was believed to be the son of Mr Conley, 48, and Valerie Jackson from a previous relationship.
“My heart goes out to the mother who received the text from her daughter that she was in trouble, and the mother was many states away but of course did what any mother would do and called 911,” Harris County District Attorney Devon Anderson said.
Mr Conley stood shackled and handcuffed in front of State District Judge Vanessa Valesquez without a lawyer at his court appearance.
Ms Anderson said it would be a few months until she decides whether to seek the death penalty.
Chief Deputy Tim Cannon of the Harris County Sheriff’s Office said: “We… cannot fully comprehend the motivation of an individual that would take the lives of so many innocent people. Especially the lives of the youngest.
“The killer’s motives appear to be related to a dispute with Valerie.”
Mr Conley is being held in jail without bond.
The officials had previously said the bodies of five children and three adults were found in the house.
Court records showed that Mr Conley’s criminal history dated to at least 1988, the Associated Press reported.
Last month, he was charged with assault of a family member, the news agency reported.