California is joining eight other states and the District of Columbia in a lawsuit to stop the release of blueprints to make 3-D printed plastic guns.
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State Attorney General Xavier Becerra blames President Donald Trump for a government settlement that would have allowed the plans to be posted online.
A federal judge has issued a temporary restraining order to block the settlement.
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The guns would be untraceable and undetectable.
The White House says the administration supports long-standing laws against owning plastic guns.
The Texas company seeking to release the plans, Defense Distributed, says it may appeal.
California joins lawsuit against 3-D printed guns