Washington state’s attorney general has promised to uncover “what truly motivated” President Donald Trump’s executive order on immigration, an approach that could prompt a rare public examination of how a U.S. president makes national security decisions.

The presidential order imposed a temporary ban on travelers from seven predominantly Muslim countries, but a federal judge has barred enforcement of the order while the court considers a challenge brought by Washington state.

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Stanford University Law School professor Michael McConnell, a former federal appeals judge, said the court should not consider campaign statements, because Trump only swore an oath to uphold the Constitution after he became president.