Justin Bieber -- Held Up at LAX Customs

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Justin Bieber's return to the United States just hit a snag at the airport -- he's being held up at customs, and TMZ has learned it's all because of his string of legal troubles.

Justin and co. flew into LAX from Japan some time this afternoon -- but his party van, his entourage and his luggage have been waiting out by the curb for about an hour now.

Sources close to Bieber tell us U.S. Customs agents are giving JB a secondary customs search ... which includes getting grilled about all his open criminal cases -- DUI arrest in MIami, egg throwing at his neighbor, etc...

We're told it's standard ops for anyone with a record like Bieber's -- and doesn't mean he's getting arrested for anything.

You'll recall he also got the 3rd degree when he landed in NJ on Super Bowl weekend.