The fire started at the Smurfit-Stone Container Corp. about 7:10 p.m. Friday in a storage facility behind the main building.
Burn patterns and progression of the fire, as well as interviews with witnesses, led investigators to believe the fire was caused by an arsonist, said Paul Vido, Bureau of Alcohol, Tobacco, Firearms and Explosives special agent, said Wednesday.
Vido said fire officials have not pinpointed any suspects, but ``there are some people we are looking to interview.''
Now that investigators have determined a cause, South Salt Lake fire crews can extinguish the final hot spots left from the fire.
``It's tough work of systematically taking out those bales of paper,'' South Salt Lake Fire Chief Steve Foote said.
The fire required engines from all of the valley's agencies, including more than 100 firefighters, said Scott Freitag, Salt Lake City Fire Department spokesman. Foam trucks also were brought in from Salt Lake City International Airport.
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