HOUSTON -- A team from the Centers for Disease Control and Prevention scoured Key Middle School, looking for whatever has been making people sick, KPRC Local 2 reported Thursday.

"There are several different factors. It's very hard to pinpoint one source," said Dr. Nancy Burton with the CDC.

Burton said the team found scattered mold, high levels of humidity, water getting i, and issues with the ventilation system. Part of the problem, she said, was improper maintenance.

Gayle Fallon of the Houston Federation of Teachers said the new findings show that the Houston Independent School District unnecessarily put people at risk by denying the problem.

"They still can't tell us what's wrong, but at least we're no longer playing the game that nothing's wrong. It's insulting to employees and endangering children," said Fallon.

Teachers, staff and students have complained about becoming ill since the school year began. Several teachers and staff members have been rushed to hospitals for treatment for headaches, chest pains, trouble breathing and nausea.

HISD spokesman Terry Abbott would not respond directly to the findings, but said now they're working to fix the problems

"You can always do things better, sure," said Abbott. "We've done a lot of maintenance on the building, and there's a lot that we'll continue to do."

The CDC asked the district to focus on specific areas: stop the leaks, clean the rest of the mold, stop moisture from coming in and balance the ventilation system.

But what about the tests HISD ran that they said showed no problem?

CDC officials said HISD's tests weren't necessarily inaccurate, they just were not broad enough to find all the issues.

"The small sampling they took looked at mold but it did not look at any other factors," Burton said.

CDC officials said they may never find an exact cause of the illnesses, but the team believes the building can be fixed to the point that it will once again be safe for children.

Burton said the CDC has gathered all of the evidence and information they needed from inside the building. They will study the findings and expect to have an initial letter to the district within a few weeks and a final report within a few months.

Meanwhile, Key teachers and students will continue to conduct classes at Fleming Middle School, where they were moved temporarily on Wednesday.