INDIANAPOLIS -- A massive, three-alarm fire gutted an under-construction six-story apartment complex a few blocks north of the Indiana Statehouse downtown Thursday morning in what veteran firefighters called the largest blaze in the downtown area in 20 years.

No one was injured, but 125 firefighters were sent to the Cosmopolitan on the Canal at Michigan Street and Senate Avenue. The 218 units at the complex were not occupied.

Indianapolis Fire Department spokeswoman Rita Reith said the buildings, valued at $28 million, were destroyed. A portion of the complex collapsed hours after the fire began, and crews were still battling the blaze hours later.

The complex was in a picturesque block along the pedestrian walkway-lined downtown canal, where several other residential and office buildings are located, on the western edge of downtown.

Charred remains and shells of structures were left behind as firefighters kept pouring water on what was left of the fire just before 9 a.m.

Developers Vow To Rebuild

The buildings included apartments that were nearly complete, a parking garage and cafeteria. The units had been expected to be ready for occupancy by in a couple of months, and developers plan to rebuild.

"Basically, I'm just totally sick and my whole construction team is devastated," said Gordon Benner, of Flaherty & Collins Properties.

The first floor of the complex was set aside for retail shops, with apartments above. The building had several levels and has six floors at its highest point. About 170 of the 218 units had been framed. Apartments were to be priced at between $1,200 to $2,500 a month.

"We have insurance. It was scheduled to open in May," said Jerry Collins, of Flaherty & Collins. "It's not going to open in May, but we will rebuild."

Firefighters Face Numerous Challenges In Blaze Battle

The fire, which began at about 3:20 a.m., sent smoke billowing across Indianapolis' skyline as firefighters worked to keep the blaze from spreading to adjacent buildings.

"It makes it pretty tricky down here with the canal and … accessing these buildings," Reith said. "There are narrow areas where the apparatus is too big to really position itself well to get to the fire."

The blaze caused some exterior damage to the Historic Landmarks Foundation building, adjacent to the apartments. The extent of that damage wasn't immediately apparent.

"While our building, which sits adjacent to the complex involved in the blaze, did suffer damage, if not for the quick work of the Indianapolis Fire Department, our home might have been lost," said Marsh Davis, president of the foundation, in a statement.

The Landmarks Foundation office is a 1879 Italianate home that the organization restored and expanded in a $2.1 million project, according to the group's Web site.

"The fact that they were able to save the building just south of it was remarkable," said Indianapolis Mayor Greg Ballard, who arrived at the scene about an hour after the fire began.

Residents Incredulous As Fire Rages

Residents were urged to evacuate nearby buildings, but most stayed behind. The downtown canal was aglow in what many called a surreal scene as flames towered high into the air before dawn. Many people heard explosions as the fire grew. The fire sent gray smoke billowing around the spotlight-lit Statehouse dome three blocks away in the pre-dawn hours.

"I've never seen nothing like this," said witness James Walker. "This is unbelievable."

Brisk wind blew burning embers through parts of downtown, igniting bushes across the street and sending noxious fumes throughout the area.

"I didn't know what the likelihood was that it was going to spread," said Tyler Nusbaum, who lives nearby. "There were still flames shooting off the roof."

The cause of the fire wasn't immediately known, but investigators hoped to go through the remains as soon as hotspots are put out.

Firefighters expected to be at the scene all day. Police closed off several downtown streets during the morning commute. That disrupted traffic to the nearby campus of Indiana University-Purdue University at Indianapolis, but classes remained on schedule.

Watch 6News and refresh this page for updates.