(CNN) -- Florida investigators are working to determine whether a 3-year-old girl who drowned in an inflatable pool was held down by her 4-year-old brother, authorities said.
"During the course of the investigation, detectives learned that the brother may have had some involvement with his sister's death," said Jimmie Flynt, a Daytona Beach police investigator.
The incident happened at a home in Daytona Beach on Tuesday evening.
The mother told police she left the children in an inflatable pool to go to the bathroom and when she returned, she found the young girl underwater.
Under state statutes, the 4-year-old boy can't face charges because of his age, officials told CNN-affiliate WESH.
Officials with the Florida Department of Children and Families have joined the investigation, Flynt said.
CNN's John Branch contributed to this report.