A fire that destroyed a six-story apartment complex under construction along the downtown Indianapolis canal last week was ruled an arson on Tuesday.

The fire gutted the Cosmopolitan on the Canal project that had been nearing completion and spread onto the roof of the adjacent headquarters of the Historic Landmarks Foundation of Indiana, causing smoke and water damage. No one was injured in the fire that began before dawn Thursday.

Federal agents, though, said they were not yet releasing any details on the specific cause of the fire as the criminal investigation continues.

"The investigators have ruled out all accidental causes and have determined, based on their review of evidence at the scene and through their investigation, that the fire was intentionally set," said Kim Riddell, a special agent with the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives.

Riddell would not say where the fire started, whether an accelerant was used or what other evidence might have led federal and state authorities to make the arson ruling. However, she said more than 30 investigators and an accelerant-sniffing dog were used at the scene, three blocks north of the Indiana Statehouse.

"The investigators on the scene methodically went through the scene layer by layer, reviewing the items they were looking at. Based on what they saw, they came to this conclusion," Riddell said.

Riddell said she had no information on the amount of damages, estimated earlier at $28 million to the apartment complex.

"It was going to be a great landmark for the city, and they've harmed that as well, so it's not just us that's been harmed, it's the city," David Flaherty, chief executive of Flaherty & Collins Properties, the apartment developer, said at a news conference.

Riddell said Crime Stoppers of Central Indiana is offering a reward of up to $1,000, and ATF is offering up to $9,000 for information leading to an arrest and conviction.

"Any decisions with regard to prosecutions in this matter, should it develop to that point, would be made by the prosecutors in this area," she said.