Residents cautiously returned to their homes, with some heeding the advice of environmental officials to replace air conditioner filters, after a fire at a hazardous materials plant.

The fire at the EQ Industrial Services plant caused no serious injuries or damage to homes, and the burning smell and potentially toxic clouds of fumes were gone Saturday.

Still, environmental officials urged residents to replace air filters, wipe off children's toys and kitchen counters, and wash clothes and bedding.

"I already bought new air filters," said Marcia Murto, whose family camped out at her office in nearby Morrisville and a hotel before returning home.

"We're back home and it seems to be OK," she said.

About 17,000 residents were urged to leave the Raleigh suburb late Thursday when the plant burst into flames. They were allowed to return starting Saturday morning, after the fire was largely extinguished and tests showed the air and water was safe.

"We've been given every assurance that it's safe for our citizens to go back home," Mayor Keith Weatherly said.

Officials continued to test the air immediately around the plant Saturday, as well as inside and around four schools in the evacuation area, town manager Bruce Radford said.

"It could have been much worse," he said, adding that the incident has taught people to be more aware of what's in their neighborhoods.

Hazel Markham, 72, whose home is within 1,000 feet of the plant, said she has lived in Apex since childhood but had no idea that hazardous materials were stored at the plant.

"I have been very frustrated and very concerned," she said as she waited in her car for police to reopen her street. "If they're thinking about putting (the plant) back, I certainly hope someone puts a stop to it."

The fire was extinguished shortly before 12:30 a.m. Saturday, although rubble continued to smolder.

Apex Fire Chief Mark Haraway said officials don't know what sparked the fire, or what specific chemicals or hazardous materials burned — the plant was a short-term routing facility where material was constantly coming and going. EQ registered with the county on an annual basis, and a company log was likely lost in the fire.

Environmental Protection Agency officials said the main concern had been volatile organic compounds, which quickly dissipate in the air. The fire department's decision to let the fire burn minimized the amount of chemicals that would have been washed into the ground by fire hoses, said James Webster, who was coordinating the EPA's efforts.

Rain Friday and Saturday also helped clean the air, he said.

A team from the U.S. Chemical Safety and Hazard Investigation Board, an independent federal agency that looks at industrial chemical accidents, was on the scene and is considering a full investigation, said Carolyn Merritt, Chemical Safety Board chairwoman.