The fire that burned for more than three hours in a North Massapequa strip mall Monday was deemed not suspicious, the Nassau County fire marshal's office said Tuesday.

The blaze destroyed the Island Recreational swimming pool supply store, burning chlorine and other pool-related chemicals as firefighters from 13 departments battled the flames. The fire was reported at 2:30 p.m. and declared under control at about 5:30 p.m., but it took more time to put out the hot spots.

Investigators are looking into the cause and awaiting test results and reports, said James Hickman, head of the fire marshal's investigations division.

Three other stores were damaged at the strip mall on Hicksville Road.

County and state authorities, including the state Department of Environmental Conservation, had been at the scene Monday because of concerns about chemicals in the air and chemical-tainted water runoff from the fire draining into the ground.

Because of air concerns, police evacuated the strip mall, which has more than 12 storefronts, and an office building across the street. Students and staff at a nearby school were told to "shelter in place."

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Authorities said tests Monday showed the air was safe, with gases from burning chlorine dissipating, Hickman said. Gases from burning chlorine can be toxic in an enclosed space, he said.

Most of the water used to put out the fire was contained in the basement of Island Recreational, authorities said.

Workers were vacuuming up the contaminated water so it could be hauled off for proper disposal, a DEC spokeswoman said Tuesday.

State officials wanted to assess any impact of water runoff from the fire, including whether chemical-tainted water had drained into the groundwater.

The DEC Tuesday did not provide other details on work and results of tests done at the fire scene.