The Mount Vernon Fire Department says air-quality tests have come back clean Saturday after a massive fire at the KEM Chemical Corporation warehouse.

The four-alarm fire at the South Fulton Avenue plant broke out around 7:30 p.m. KEM is a specialty industrial plant and laboratory supply distributor. The plant contained sulfuric acetones, creating purplish chemical flames that proved difficult to fight.

The fire prompted authorities to evacuate surrounding blocks and tell residents nearby to close windows and stay inside. The fire spread to two other buildings in the largely industrial area. No injuries were reported.

A demolition company was on site Saturday to knock down the KEM building as it was deemed structurally unsafe. Con Ed crews turned off power to the entire block, and the gas line leading to the warehouse was also shut down.

The fire is under investigation, but due to the extensive damage to the building, officials say they may never be able to pinpoint the exact cause.