The fire started before 2 a.m. near De Anza and Central avenues, according to the Riverside Fire Department. Officials say it started at the back of the building and then spread.
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When crews arrived at the scene, they found a dumpster and a trash can also engulfed in flames in the same area.
The post office sustained about $50-70,000 in damage, but officials said none of the mail was destroyed. The portion of the building that burned contained pre-paid envelopes and shipping boxes.
The fire is being investigated as arson, but an arrest has not been made. A person of interest had been detained for questioning but that individual was later released.
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Investigators are reviewing several cameras in the area for any leads on a possible suspect.