Yuma police said 26-year-old Alexis Keslar jumped into the steep-sloped canal Friday but couldn't rescue the boys because the deep water was moving too fast.
Police on Tuesday issued a statement detailing how the twins ended up in the canal and said the Yuma County medical examiner determined that the deaths were accidental drownings.
"We're not seeing any criminal actions or activity," police Lt. Don Willits said.
He said Keslar managed to get out of the canal, but she'd lost her cellphone in the water. She yelled for help, and a Yuma city public works employee who was nearby heard her and called 911, Willits said.
It took authorities more than an hour, however, to find and retrieve the boys.
The canal's water level was lowered to help the rescue effort, but Silas and Eli Keslar were pronounced dead at a hospital.
"When I found out who the little boys were, all I could feel was a knot in my stomach," Marlene Gleim, a friend of the twins' grandmother, told KYMA-TV.
Copyright 2015 The Associated Press modified.