Texas police are trying to figure out how a 15-month-old girl and her twin sister remained underwater in a bathtub long enough until they stopped breathing and eventually died, My Fox Houston reported.

Twins Savannah and Sabrina Russo were flown to Memorial Hermann Children's hospital after their mother found them submerged and unresponsive in the tub at their Pearland home on Feb. 17. Savannah died the same day while her sister was placed on life support.

But on Monday Pearland police said Sabrina has also died.

"The 15-month-old infant was taken off of life-support devices at approximately 11 p.m. on Sunday...and was declared deceased shortly thereafter," the Pearland Police Department said, according to My Fox Houston.

Details on what happened in the moments before Barbara Russo found her daughters in the tub are still hazy. Russo and the twins' uncle were the only adults in the home at the time, police said. The mother called 911 to report the suspected drowning.

Autopsies are to be performed on both twins while police and Brazoria county prosecutors determine if their deaths were accidental or criminal.

"It's very sad, not one baby, but two," neighbor Lene Owerbach told My Fox Houston. "I don't even know what to think it's going to be with them the rest of their lives. It must be devastating to them," she said of the grieving family.