The woman shot her 19- and 17-year-old daughters on Friday in the Westheimer Lakes-North neighborhood near the Houston suburbs of Fulshear and Katy, the Fort Bend County Sheriff's Office said.
After someone called 911, a Fulshear police officer shot the mother when she refused to put down her weapon, the sheriff's office said.
The daughters were found on the street with gunshot wounds. The 19-year-old died at the scene; the 17-year-old died at a hospital, the sheriff's office said.
The woman's husband was taken away by paramedics, but was not injured, CNN affiliate KTRK reported.
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Sheriff Troy Nehls said deputies had gone to the family's home in the past, but he declined to provide additional details.
"It would be too premature to give you a motive as to why something like this took place," Nehls said.
"The best we can do is pray for the father and husband so he can work through this," the Sheriff added.