Can a U.S. citizen arrested in this country on terrorism charges be detained without trial? As with other aspects of the war on terror, the answer isn’t clear. Sen. Dianne Feinstein, D-Calif., has proposed important legislation that would end the ambiguity and explicitly prohibit indefinite detention in such cases.

A defense authorization bill signed by President Obama last year authorized the military to detain any “person” who supported al-Qaida, the Taliban “or associated forces.”

But it also said that nothing in the bill “shall be construed to affect existing law or authorities relating to the detention of United States citizens, lawful resident aliens of the United States or any other persons who are captured or arrested in the United States.”

In signing the bill, Obama promised that his administration “will not authorize the indefinite military detention without trial of American citizens.”

But there is no guarantee that Obama will be president after this year’s election, which means that U.S. citizens would be protected only by the “existing law” referred to in the defense authorization.

And there is at least one source for the proposition that U.S. citizens could be detained without trial under the laws of war: a 2004 Supreme Court decision called Hamdi v. Rumsfeld.

Unfortunately, despite the proven success of the civilian justice system in trying terrorism cases, Congress continues to thwart the administration’s original plan to hold civilian trials for detainees now held at Guantanamo.

Congress should establish one system of justice for all suspected terrorists, but the Feinstein bill would at least codify Obama’s promise that no U.S. citizen will be held without trial.