Police spokeswoman Tania Rues said authorities received a 911 call around 7 p.m. Wednesday by a man who had either seen or heard what happened.
WFOR-TV, Miami, said police went to The Ashlar Apartments after receiving the 911 call and are now trying to discover just what happened.
"We did have someone who called this in and at present time police are meeting with this individual and speaking to him on the matter," Rues told reporters.
She refused to provide details about the initial investigation, but said the unnamed victim knew the man who made the call. She also declined to elaborate on the nature of their relationship.
Miramar police were trying to determine if the 2 1/2-year-old toddler fired the shot that killed his mother.
"It's really sad," said neighbor Abel Hernandez. "I feel bad for the little kid. He's not gonna see his mom."
Hernandez said he and several other people in the apartment complex were looking for a missing child in an unrelated incident when the shot rang out. Hernandez said he saw a female officer run to the apartment and when she came out she had the toddler.