WALTHAM, Mass. (WHDH) — Investigators said a massive fire that destroyed an apartment building on Elm Street in Waltham last month was “intentionally set.”

In a press conference Thursday, officials announced that fire, which reduced the 264-unit building to rubble on July 23, is now being investigated as a case of arson.

Four firefighters were injured battling the flames. Crews from 19 cities and towns responded to assist when the fire broke out at around 4 a.m. Damage was estimated to be more than $100 million.

Waltham Police concluded that the cause was arson following a “large-scale investigation,” officials said.

Investigators are asking for the public’s help as they work to crack the case. A $100,000 reward is being offered by contractors who own the property in exchange for information that leads to an arrest and conviction.

“When someone commits a criminal act, I think it angers everybody because these are acts we should not have to endure,” said Waltham Fire Chief Paul Ciccone. “There’s a ripple effect here that was caused by a crime.”

The wood frame building was under construction when the fire broke out.

Critics said the city should never have approved a wood construction of such a size. It is unclear how long it will take before crews start to think about rebuilding.

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