Investigators found sellers on social media and pretended to be buyers.
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So far, they have confiscated 300 pounds of illegal fireworks, worth thousands of dollars. Four people have been arrested.
RELATED: Reward money of $2,500 for reporting illegal fireworks in East Bay
The Contra Costa County Fire Marshall says a fire earlier this month shows what can happen.
"A fire that occurred in Antioch, where an apartment complex ended up burning down displacing 40 residents from just a single firework going off," explained Fire Marshall Chris Bachman.
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One person was arrested in that fire.
The department is asking people to phone-in tips anonymously.
VIDEO: Fears of fireworks igniting fires in Bay Area before July 4th
Unfavorable conditions raising fears of fireworks igniting fires in Bay Area before July 4th
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