After reviewing video surveillance fire investigators say the blaze was stated by materials used to clean and resurface the wooden gym floor.
Rags thrown away by district employees spontaneously combusted.
"Yes they were improperly disposed of," Lieutenant Nick Craig of the Green Bay Fire Department says. "Did they do it on purpose? Obviously not."
This week a team of roughly 400 people from Paul Davis restoration will work 24 hours a day to get students and staff back in the building.
"The smoke travel throughout the entire building," J. Murphy of Paul Davis Restoration says. "We have to address every room cabinet closet cavity in the building in one way or another."
Sporting equipment, band instruments, and technology items all need to be cleaned. It is not known if everything can be salvaged.
"I'm happy to say we are about 90% complete with the cleaning of all the books the shelves," Murphy says.
Even when students are allowed back inside, Preble's Principal Natasha Rowell says they won't have a gym to use.
"The gym will be isolated that there is no cross-contamination," she says.
However to ensure students can compete in fall sports the goal is to have the locker rooms open later this week.
"We will build to isolate those areas from the rest of the school and create a separate entrance for the students," Murphy says.
By the end of the week the district hopes to announce a start date for the school year. We asked about the total cost of this damage - but so far that number is not being made public.
The company in charge of restoration, Paul Davis National, says smoke and soot damaged the entire building.
A spokesman says it needs to be cleaned top to bottom.
A team of roughly 100 people started the process on Friday.
Once the total damage assessment is made, they will add a night shift.
They are hoping to work 24 hours a day, by the middle of this week.
The school district is holding a press conference this afternoon at 2:30.
In addition to representatives from the school and restoration company, the Green Bay Fire Department will be on hand.
Click here to find updates on the cleaning process, school starting dates and more.
Local 5's Millaine Wells will have the latest in live reports starting at 4:00 pm.