In the last two weeks, the department began an in-depth review of the backlog, De Cossio said in an interview.
"Does 300 sound like an appropriate number? I would say no, we wouldn't target 300 as typical open cases to have," De Cossio said. "We need to go case by case, open up each report, and review the status."
Some arson investigators who began probes into fires in the past have been transferred to other parts of the department, De Cossio said. The department plans to pull those former investigators from their current posts -- temporarily -- to complete reports, he said.
De Cossio wants to know whether the department has enough fire investigators on staff, whether they have enough training and whether they're being called to more fires. Currently, the division has seven full-time investigators, though some of them are new and still getting training.
Michie Wong, the department's recently retired fire marshal, said the arson task force "is incredibly understaffed." She notes that before recession and citywide budget cuts, there were a dozen full-time investigators.
The man who runs the arson task force has repeatedly told the Fire Commission his unit is understaffed.
In February, Capt. John Darmanin said his unit was "woefully short" of personnel. And, again at the last commission meeting earlier this month, Darmanin delivered an emotional plea to the commission for a supervisor's position he said the department has not filled.
"I can only do so much," Darmanin said. "I work a lot of hours on my own time because that's the only way to get it done. We need help."
Wong, an assistant deputy chief, stepped down June 5 after this year's series of fatal fires prompted criticism over how the fire marshal's office had documented safety problems in the city's apartment buildings.
In the wake of the fatal 22nd and Mission fire in January, Wong acknowledged her staff did not properly document inspections for the building that went up in flames. That structure did not have sprinklers on its top two floors, two of its exits were blocked and the apartment's smoke alarms did not operate correctly.
"The recent fires definitely put some pressure on me," Wong said in an interview. That pressure, her belief that her division did not have enough inspectors and investigators, and her own personal medical issues prompted her to retire earlier than planned.
"The next fire marshal maybe can have the staffing they need, can build the staffing they need, to run the division as I feel it should be run," Wong said.
Wong's two-year tenure as the city's top fire inspections and investigations official came as the department scrambled to keep up with the city's boom in construction. The potential dangers from the increase in construction came to light after a massive fire in Mission Bay in March 2014 and another blaze that injured eight workers at the old Renoir Hotel the following September.
With Wong as fire marshal the Fire Department penalized construction companies associated with both fires. San Francisco's development explosion also had Wong pushing for the city to hire more fire inspectors.
Wong's retirement ends a career at SFFD that began in 1997. She started as a firefighter emergency medical technician and moved to the Bureau of Fire Prevention in 2002, gradually moving her way up.