Tuesday in the debris, which contained portions of the roof and interior of one of several warehouses on the sprawling mill property off Route 101.
The warehouse is one of several that has been torn down by a demolition company in the last week.
The complex of about nine buildings and other structures, is being demolished to make way for a 500,000-square-foot shopping center, equal in size to Lisbon Landing, that is being built by a Fairfield developer.
"We're still looking at some things before we can make a determination on how the fire started," Fire Marshal Ray Allen said Wednesday.
He said investigators from the state Fire Marshal's Bureau will return to the site today. He said he hopes to have more definite answers today.
Allen said the cause of the fire remains "undetermined." He refused to say if investigators suspect it might have been set.
Firefighters from Dayville and five other Killingly departments battled the fire for more than 90 minutes Tuesday before bringing it under control.
Meanwhile, Killingly's deputy fire marshal, Donald Burton, is investigating a small, but smoky fire that broke out in an apartment on Hawkins Street in Danielson at about 11 p.m. Tuesday.
Burton said the fire was contained to an area around a window in the apartment at 41B Hawkins St., occupied by Crystal Kreimes.
He said Kreimes discovered smoke in the hallway leading to her apartment as she arrived home.
"When she opened the door to the apartment, the room was filled with smoke," Burton said. "That tells me the fire smoldered for some time before it started to flare up."
He said building manager Don Baker, who lives in another apartment, was able to douse the fire with a bucket of water, holding it in check until Danielson firefighters reached the scene.
Burton said the cause of that fire is also undetermined, although he's leaning toward ruling it an accident. He said he is continuing to investigate.