Investigators ruled the fire that caused extensive damage to the Cedardale Health and Fitness club was accidental.
They said the fire started in a women's locker room early on the morning of March 1.
After using heavy equipment to remove debris from the site, authorities said they found several possible ignition sources for the fire, such as electrical wiring, small appliances and light fixtures.
In a statement released last week, State Fire Marshal Peter Ostroskey, Haverhill Fire Chief William Laliberty and Haverhill Police Chief Alan DeNaro said there is not sufficient evidence to point to one source over another as the most likely cause of the fire.
The statement said nothing was found to indicate the fire was intentionally set. At the time of the blaze, the fitness club was unoccupied, except for a maintenance man who got out of the building safely.
It took firefighters about four hours to get the blaze under control, but after that areas in the building where small fires continued to burn, commonly known as "hot spots," kept popping up. Firefighters repeatedly poured water onto the hot spots well into the afternoon, with Laliberty attributing the cause of those spots to collapsed flooring which was trapping heat.
The next day, strong winds prevented firefighters from beginning an extensive investigation into the cause. It wasn't until early this week that investigators were able to get a full look at the inside of the building.
Fire officials estimated that in total, the blaze caused between $2 million and $5 million in damage to the 931 Boston Road club, which is still closed to its 5,000 members who use the facility to swim, play tennis and basketball, and participate in Crossfit, among other activities
Dozens of employees are out of work or have found temporary positions elsewhere.
City officials said they are helping Cedardale's owners look for a temporary location for the club. Local fitness centers have stepped up to offer Cedardale members reduced-cost or free temporary memberships, while others have also offered temporary jobs to Cedardale employees.
In the immediate wake of the fire, the club's owners, the Veasey family, pledged to rebuild the damaged wing of the complex, and have been communicating with club members via Facebook.
The club has told members it was in the process of salvaging computers and essential items from the building in preparation for establishing a temporary business office.
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