The incident happened at about 4:15 p.m. Saturday.
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Video from an Eyewitness News viewer posted to Twitter shows the four masked people, dressed in black, running in the Puente Hills Mall.
The sheriff's department says the gang used a hammer to smash glass display cases in Daniel's Jewelers.
The suspects took off with an estimated $25,000 to $30,000 dollars worth of jewelry. They remain at large.
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No further details were immediately known.