(CNN) -- All four people found dead inside tennis great James Blake's burning mansion had been shot, investigators said Thursday.
Blake was not at the Florida home at the time of the blaze Wednesday and was leasing out the property, Hillsborough County Sheriff's Col. Donna Lusczynski told reporters.
The medical examiner is working to determine the cause of death for two adults and two teenagers, apparently from the same family, Lusczynski said.
Authorities haven't identified the victims, but they say Darrin and Kimberly Campbell lived in the house with their children, Megan and Colin. The Campbells had been renting the 5,800-square-foot house from Blake for the past two years.
Investigators found fireworks throughout the home, and a surveillance video obtained by CNN shows a man believed to be Darrin Campbell inside a Phantom Fireworks store in Tampa, Florida, on Sunday, buying about $600 worth of fireworks. He walks out of the store with one cart, and an employee seems to help him out with a second.
He bought several gas cans that same day at a home improvement store, and a firearm registered to him was found in the residence, Lusczynski added.
On Wednesday, Lusczynski said the fire had been "started intentionally by an unknown accelerant."
The Hillsborough County Sheriff's Office would not elaborate on what they believe happened or on a possible motive.
But investigators say they are not searching for a suspect.
Firefighters first learned of the blaze in a 911 call around 5:45 a.m. Wednesday reporting an explosion and flames.
Blake, 34, born in Yonkers, New York, starred at Harvard before breaking into professional tennis. He peaked at No. 4 in the world rankings in November 2006, according to his ATP World Tour profile.
He retired from the sport last year, and remains active with his namesake foundation and Thomas Blake Sr. Memorial Research Fund to support research at Memorial Sloan-Kettering Cancer Center.
CNN's Susan Candiotti contributed to this report.