They found three adults, two women and the suspected gunman, dead and a 15-year-old boy suffering serious gunshot wounds at the property.
The juvenile was treated by paramedics at the scene and taken to hospital where he died.
The victims were the wife, mother-n-law and stepson of the shooter.
At a press briefing, Casselberry Police Chief Larry Krantz said they were “clueless” why the triple homicide had occurred.
“We don’t have a history of violence with these individuals,” Mr Krantz said.
“We just don’t have an understanding as to why this happened.”
He said the gunman was from Cuba and had lives in the United States for about 15 years.
Mr Krantz said officers had responded to a call from the gunman’s wife on 9 June as she was concerned that her husband had purchased a firearm.
Police confirmed that he bought a weapon legally a week ago from a local gun store.
Mr Krantz said the police were aware of rumours that the gunman had purchased the weapon in response to several recent mass shootings.
He added that there had been several unreported incidents of domestic violence at the address. It’s unclear how police were aware of these incidents.
No names have yet been released as police are trying to contact next of kin.