Riverside city officials say the cache packed enough power to cause damage similar to the 2021 explosions that rocked neighborhoods in Ontario and South Los Angeles.
Both incidents were caused by illegal fireworks.
Ahead of the 4th of July holiday, Riverside city officials are warning residents that all fireworks - even "safe and sane" ones - could land them with a $1,500 administrative citation.
"Fireworks are illegal 24/7, 365 days a year. So, even though 4th of July is the most busy for the city we will cite and issue an administrative citation any time, any day of the year," said arson investigator Capt. Ray Mendoza.
A joint task force of fire, police and the fire marshal's office is on the lookout patrolling neighborhoods for firework activity.
If a hefty fine isn't enough of deterrent, they hope to send a strong message demonstrated with a homemade explosive or M80 strapped to a watermelon.
"One of these homemade destructive devices can land you in prison for two, four or six years but it is also mandatory that you would serve 90% of that sentencing," said Police Chief Larry Gonzalez.
Fireworks are not only illegal in Riverside they can cause dangerous fires.
Fire Chief Michael Moore says nearly half of all fires that are reported each July 4th can be traced back to fireworks.
"The danger associated with these fireworks is immeasurable. The danger extends to families, communities - including not limited to our firefighters who are responding to these incidents," said Chief Moore.
The city is encouraging residents to report any firework sightings to its 311 app or by calling the number.
But just because fireworks are not allowed, it doesn't mean you have to miss out. Riverside and other cities have free firework displays put on by the experts.
"Head to the park not the firework stand," said Riverside Mayor Patricia Lock Dawson.