INDIANAPOLIS -- Federal fire investigators dug through a massive mound of rubble Saturday that was left behind when an under-construction, upscale apartment complex was gutted downtown earlier this week.

Twenty investigators from the Bureau of Alcohol, Tobacco, Firearms and Explosives are trying to determine what caused the fire that destroyed the Cosmopolitan on the Canal complex Thursday morning, 6News' Sarah Cornell reported.

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The smell of smoke lingers in the air near the scene of the blaze at Michigan Street and Senate Avenue, which scarred the landscape of the normally picturesque canal.

Judging by the aftermath of the fire, it is hard to imagine that the mangled, charred mess left behind was going to house 210 apartments, with monthly rent at between $1,200 and $2,500, and retail shops. Developers had planned to open the complex to tenants by May.

"It was a nice facility. It really improved the downtown area a lot, I thought," said Gary Creed, who lives nearby.

Investigators are charged with the daunting task of sifting through mountains of rubble to come to a conclusion on what parked the blaze. The investigation could take quite some time.

"This will be a multiple-day investigation. It's very much like an archaeological dig, where we will go through layers and layers to get to the bottom of where this fire actually started and, hopefully, what the cause of this fire is," said Kim Riddell, of the ATF.

Investigators had to put together a plan of attack, determining which piles of debris to sift through first. What seems like an overwhelming job to a layman is much easier for the trained eye.

"They use tools, such as the accelerated detection K-9, to help them focus, as well as just analyzing what they see based on their knowledge and experience," Riddell said.

Rumors that someone intentionally set the blaze swarmed around the rubble in the days following the blaze, but investigators said any speculation is much too early.

"We will leave no stone unturned. It will be a thorough investigation, and we will be here as long as it takes," Riddell said.

Investigators were also gathering pictures and interviews from witnesses in hopes they might unlock information crucial to their work.

The complex was valued at $28 million, but was worth about $37 million, including the value of the land. Property owners said they plan to rebuild.