Bodies in charred home of ex-tennis pro

• NEW: Fire was "started intentionally by an unknown accelerant," official says


• NEW: Two victims had upper body trauma


• NEW: Two adults are found in one bedroom, and teenage children in their own bedrooms


• Former tennis pro James Blake, 34, owns the house but was leasing it

(CNN) -- Four family members' bodies were found after an intentionally set fire damaged a Florida home belonging to former tennis player James Blake, who was leasing it out, authorities said Wednesday.

"We have also confirmed that the fire was started intentionally by an unknown accelerant," Hillsborough County Sheriff Col. Donna Lusczynski told reporters.

Two bodies had upper body trauma, Lusczynski said.

The four apparent family members -- two adults and two teenagers -- were found in their respective bedrooms after the early morning fire, Lusczynski said.

Identifying the bodies will take several days, she said.

The adults were in one bedroom, and each teen was in a separate bedroom, Lusczynski said.

Authorities found small commercial-type fireworks in the Tampa home, officials said.

Blake was not at the home at the time of the fire, Lusczynski said. He was leasing the property and was contacted by phone after the incident by sheriff's officials.

Blake, 34, was once ranked No. 4 in tennis among world singles and played in the 2006 Tennis Masters Cup, according to his Facebook page.

At 5:45 a.m. Wednesday, authorities received a 911 call about an explosion and then a fire at the home.

Firefighters began extinguishing the fire and then discovered two bodies inside the 6,000-square-foot house, authorities said. Crews found a third body and then a fourth, the sheriff's office said.

"Due to the magnitude of the structural damage caused by the fire, this investigation will be a slow, meticulous process," the sheriff's office said.

CNN's Marlena Baldacci contributed to this report.