The hazardous waste storage plant fire that led to the evacuation of thousands of residents in early October may have started where chemicals such as pool chlorination tablets were stored, investigators said Monday.

An initial review found that a chemical cloud was seen above that area before the fire was detected on the night of Oct. 5, according to a statement by the U.S. Chemical Safety Board.

A lack of physical evidence surviving the blaze at the EQ Industrial Services Inc. plant means the source of the chemical cloud likely will not be found, the federal agency said. However, witness accounts are consistent with an uncontrolled reaction involving incompatible chemicals, it said.

The fire quickly grew out of control, lighting up the sky with explosions and blanketing parts of the town in a yellow-green haze. Town officials urged as many as 17,000 people to evacuate because of the potential for toxic fumes and a few dozen people sought medical attention.

Residents who left were not allowed to return home until firefighters extinguished the blaze two days later.

A task force will examine regulations for hazardous waste storage centers and recommend changes to tighten those rules, the agency said.