“We systematically eliminated all possible causes,” said the state’s fire marshal, Peter J. Ostroskey. “That led us to this point.”

Initially, fire officials had said they did not believe the 10-alarm fire was suspicious. But once they began investigating the charred remains in detail, they began to rethink the cause, Fire Chief Paul Ciccone said.

WALTHAM — The massive fire that destroyed a big apartment complex under construction in Waltham in July was caused by arson, investigators said Thursday, with damages estimated at $110 million.

The blaze began in the early morning on Sunday, July 23, and demolished the five-building complex on Cooper Street near the Charles River. The 264 units were scheduled to open next year.


Citing the ongoing investigation, neither Ciccone nor Ostroskey would shed any light on a specific cause or potential motive for setting the fire. Nor would they reveal if they have suspects.

“We’re still developing information and collecting evidence,” Ostroskey said. “It’s still an active investigation; we’re not going to give out any additional investigative details.”

Officials did, however, ask that anyone with information about the fire share it with investigators. “We need the public’s help to solve this case,” Ostroskey said.

On top of a standard $5,000 reward for information that helps solve arson cases in Massachusetts, the project’s owner, a group led by Lincoln Property Co., and general contractor Callahan Construction Managers are offering a $100,000 reward upon the conviction of whoever is responsible.

“This is an inexcusable crime, and Callahan will not tolerate acts that put the safety of the community, first responders, and our job sites at risk,” said Lisa Nickerson, a spokeswoman for the construction company.

Nickerson also declined to comment on potential motives.

Lincoln, based in Dallas, is a large commercial real estate firm that manages about two dozen apartment complexes in Greater Boston, as well as a number of office buildings. Bridgewater-based Callahan is one of the region’s leading builders of mid-rise apartment buildings, particularly in and around Boston.


At $110 million in damages, the fire would be among the costliest on recent record in Massachusetts. Malden Mills collected a $300 million insurance payout when its Lawrence factory burned down in 1995, but claims of that size have been few.

In Massachusetts, nearly 17,000 fires in buildings in 2016 caused an estimated $235 million in damage, according to the Division of Fire Safety. Of those, 151 were determined to be caused by arson.

State figures indicate the number of arson fires has fallen by half over the last decade.

Nickerson said that Callahan was insured and plans to restart construction as soon as possible.

The contractor has increased security at many of its projects, she said, adding patrols and security cameras to supplement existing measures.

The construction site is on the Charles River at the edge of downtown, near a strip of popular restaurants on Moody Street.

The Waltham fire, as well as one in June that burned down a complex being built in Dorchester, has raised concerns about the safety of the many all-wood mid-size apartment buildings going up in the region.

Boston officials have concluded that the cause of the Dorchester fire was the faulty installation of an exhaust vent.

But the all-wood structures, according to some fire safety specialists, are especially vulnerable to fast-moving fires during construction, before sprinklers and other safety protections have been installed.


While state fire codes allow municipalities to mandate extra safety steps, such as off-hours security, towns rarely do.

Now, Waltham City Council President Diane LeBlanc said the city should more closely review the safety protocols of any large project and consider requiring additional security.

“Maybe they should have more cameras. Maybe they should have more signs. Maybe they should have night watchmen,” she said. “To the extent the City Council could have that conversation, I don’t think that’s unreasonable.”

More substantial toughening of safety rules on apartment construction would have to be made through changes to the state building and fire codes.

Ostroskey said he has convened a working group of fire and building specialists to study how best to safeguard wood-frame projects, which have become increasingly popular among developers because they are less expensive to build than one using steel.

He declined to advance any recommendations Thursday.

“We don’t want to get in a referendum on wood-frame construction here today,” Ostroskey said. “We’re here to talk about the investigation.”

State Fire Marshal Peter J. Ostroskey announced that arson was the cause of the 10-alarm fire at 20 Cooper Street. in Waltham. Jessica Rinaldi/Globe Staf

Travis Andersen of the Globe Staff contributed to this report. Tim Logan can be reached at tim.logan@globe.com. Follow him on Twitter at @bytimlogan.