He said that by next June, he wants the backlog reduced to around 40 or 50 cases, and he wants it to be the department's practice to complete investigations a month after they're begun.
"We have a very good team in place and the numbers reflect that," De Cossio said. "We are trending in the right direction, those numbers are declining, and I anticipate in the next six months or so we should be pretty close to our target."
Campos said a hearing to look into why investigations are taking so long to complete will probably take place as the Board of Supervisors considers recommendations a city task force unveiled last month that call for stricter standards for fire alarms in older apartment buildings.
The task force, which includes De Cossio, was created after two fires in the Mission District killed three people and displaced scores of others.
Campos said he asked Fire Chief Joanne Hayes-White a few weeks ago to provide information to ease residents' concerns that the rise in fire activity in the quickly gentrifying neighborhood was the result of arson.
The Fire Department's data show there have been 19 structure fires in the Mission this year, none of which were intentionally set. The department says arson was involved in only one of the more than 60 structure fires in the district over the past four years.
Campos says that report is a good start, but not enough.
"They have to do a better job of providing information to the public," Campos said. "It's not enough for folks in the Mission for the Fire Department to say, 'Don't worry, it's not arson.' "
Campos wants the department to release the investigative results on each of the fires in the neighborhood this year.
"What we have seen is at least the appearance of dysfunction in terms of the arson unit," Campos said, alluding to complaints about the detachment's previous understaffing. "The longer you wait to say what the specific cause was, the more questions people have."
Capt. Attica Bowden, recently appointed to head the arson unit, says the department requires investigations for all of the city's largest blazes -- those in which two or more alarms have been called. For smaller fires, Bowden said, arson investigators are called in only when incident commanders believe a blaze has been intentionally set or they can't determine the cause themselves.
Of the 19 structure fires in the Mission this year, De Cossio says 10 have been the subject of arson investigations.
One of those probes is still open, concerning a March apartment fire that killed two people at 24th Street and Treat Avenue.
Eight of the other larger structure fires this year in the Mission were found to be accidental, De Cossio said. Fire investigators were unable to determine the cause of one other blaze.
Investigators were not called to the scenes of nine smaller structure fires in the Mission this year because incident commanders deemed it unnecessary. De Cossio said he feels assured those incidents were not the result of arson, though he hasn't actually looked at the cases.
Campos, meantime, is the second member of the Board of Supervisors to show concern about the backlog.
In July, board president London Breed called for Hayes-White to be replaced because of the department's failure to close out investigations.