INDIANAPOLIS -- Investigators from the Bureau of Alcohol, Tobacco, Firearms and Explosives are in Indianapolis Friday to begin work to determine a cause of a massive fire that gutted an under-construction upscale apartment complex downtown Thursday morning.

It could be several days before investigators are able to pinpoint a cause of the blaze that destroyed the $37 million Cosmopolitan on the Canal complex at Michigan Street and Senate Avenue.

Firefighters were still at the scene Friday, putting water on remaining hotspots. A crane was brought in to begin moving some of the heavy rubble so firefighters could get better access to hotspots.

Owners and fire investigators said they want to determine what caused the fire as quickly as possible, but cautioned it will take time.

"It could be days or even weeks before they come up with a final decision. There's just a lot of debris that needs to be moved," said Indianapolis fire Capt. Rita Reith. "They have a lot of square inches they have to cover and then they have a lot of interviews they still need to do."

The complex, scheduled to open in May, was two weeks from having its sprinkler system activated.

"It would have put the fire out primarily in the areas that were done," said Jerry Collins, of Flaherty & Collins. "It would have done its job."

Some of the 210 units, priced at between $1,200 and $2,500 a month, had already been reserved.

Firefighters said they think the blaze began on the west side of the complex, which was still smoldering Thursday night from a massive melted mound of debris.

"From the front side, it doesn't look like it's that bad, but when you come around and look at it from this side, it's totally gone," said Larry Lynch, who lives downtown.

Property owners said they were fully insured and plan to rebuild along the scenic downtown canal.