A massive six-alarm blaze tore through a California warehouse early Tuesday and authorities said an employee is in custody, fire spread rapidly, FOX 11 reported.
By about 5 a.m., firefighters shifted to a defensive strategy, positioning ladder trucks around the building and dousing the structure from above as the hours-long fire continued to burn, KTLA reported.
About 20 employees were inside the warehouse when the fire broke out, but all escaped without injury, according to FOX 11.
Ontario Fire Department Deputy Chief Mike Wedell said that while the building’s internal sprinkler system was operational, the fire grew "exponentially very quickly," forcing initial interior crews to retreat and adopt a defensive strategy, FOX 11 reported.
In an update Tuesday morning, the Ontario Fire Department said more than 140 firefighters responded to the blaze, which has been contained to the building as crews continue battling the fire in a defensive mode.
Authorities also warned that smoke and ash may impact air quality in the area, advising residents — especially children and seniors — to remain indoors and avoid the area if possible.
Fire officials said the incident is being investigated as an arson case and that one suspect is in custody, the outlet reported.
"At this point, we do have a subject of interest," Ontario Fire Department Deputy Chief Mike Woodell told KTLA. "That individual has been brought into custody, arrested, and taken to the police department. The individual is awaiting questioning by Ontario Police Department detectives who are en route."
Fox News Digital reached out to Ontario's police and fire departments for comment.