A blaze that destroyed a large apartment complex under construction in suburban Boston has been ruled arson.

The 264-unit complex in Waltham caught fire July 23. Fire Chief Paul Ciccone estimated the cost of the damage at $110 million.

State Fire Marshal Peter Ostroskey said Thursday that investigators have ruled out all potential causes except arson. He asked for the public’s assistance in solving the case.

The building owners are offering a $100,000 reward for information leading to a conviction on top of a standard $5,000 reward offered by the state. Officials did not cite a potential motive for the arson.