Police and paramedics responded to a home on the 900 block of West 50th Place just after 7:30 a.m. and found the girls inside a bathtub in a standup shower.
Police say it was not a bathtub large enough for both children to have accidentally drowned. Both children were in cardiac arrest as they were being rushed to California Hospital Medical Center. Paramedics were only able to revive the 5-year-old.
After talking to relatives, police believe that the mother, 32-year-old Lorna Valle, was trying to kill her children.
"The circumstances, we're still trying to sort out. But it appears now that this was an intentional act committed upon these two children by the mother," said LAPD Cmdr. Andrew Smith.
The mother was arrested and held on $1.5 million bail. She is currently on a three-day psychiatric hold at Harbor-UCLA Medical Center.
Neighbor Dina Cevallos said she went outside to move her car when she heard the father of the girls screaming. When she asked him what happened, he told her that the mother of the girls was depressed.
The 5-year-old girl was transferred to Children's Hospital Los Angeles, where she was upgraded to serious but stable condition from critical condition.
Latest word from hospital as of 3 a.m. Thursday is that 5-year-old is in critical but stable condition.