INDIANAPOLIS — Hundreds of people were allowed to return to their homes Sunday morning as Indianapolis firefighters watched over the smoldering ruins of a sprawling warehouse where a fire had unleashed a tower of billowing black smoke, raising concerns over possible toxins.

But investigators aren't likely to enter the charred wreckage until Monday, after heavy equipment clears a safe path through the piles of debris, said Capt. Rita Burris, the city's fire department spokeswoman.

"The investigators can't even get in there yet," Burris said Sunday. "They'd be climbing over tires to get at anything, and we're not going to send anybody into a dangerous situation."

More than 200 firefighters, police officers and medics responded to Saturday's massive blaze, which was fed by thousands of square feet of recycled tires, wood pallets, shingles and siding. The fire caused an acrid black plume that could be seen for miles.

The fire was brought under control by 9 p.m., but crews stood watch overnight in case hot spots flared up again. Four firefighters suffered minor injuries.

Although residents in the five-block area that was evacuated were allowed to go home Sunday, officials cautioned that the building's rubble could smolder for days and trouble those with respiratory problems. Officials urged such people to keep their windows closed or just stay away.

The most dangerous chemicals, such as carbon monoxide, sulfuric acid and sulfuric dioxide, didn't spread throughout the neighborhood as some had feared.

"Those were the ones that they were looking for and they found those right at the fire, but they didn't find any at the homes," Burris said.

Rain was forecast for Sunday, and Burris said that should help wash the particulate matter out of the air.

Burris said it could take weeks to determine the cause of the fire.