Police at the house said the bomb was intended for the family and was not a random incident. The mother opened the package in the kitchen near the rear of the house and the powerful blast ripped the front doors from their hinges, blew out windows and scattered debris into the family's swimming pool, police said.
There were seven other adults and children inside the house at the time of the mid-day explosion. Their identities were not released, but police said one of the adults was the boyfriend of the victim.
Investigators from Plantation police, the bomb squad and postal inspectors gathered evidence at the house to help determine what kind of bomb was used. The package was left on the porch and brought inside by a visiting 11-year-old boy.
A spokesman for the federal Alcohol, Tobacco and Firearms agency said police were treating the case as a murder investigation.
The woman, who was divorced, died almost instantly from the blast, said police, who have not released her name.
Her son, aged 13, and daughter, 9, were taken to hospital for treatment of minor cuts and wounds. A playmate who was visiting the house was also treated for minor injuries.
Neighbours said the divorced husband of the dead woman was believed to be out of the country. Police were trying to locate him for questioning.