WASHINGTON — U.S. Attorney General Eric Holder said some prisoners already serving time should benefit from a new law that lowers sentences for crack cocaine offenses, but only if their crimes did not involve weapons and they do not have lengthy arrest records. Holder was testifying before the U.S. Sentencing Commission when he made the statements. Last year, Congress passed and President Obama signed the Fair Sentencing Act, which reduces penalties for crack cocaine offenses to reduce the disparity with powder cocaine penalties. The act addressed new cases but not old ones. The commission is considering whether to apply lower sentencing guidelines resulting from the new law retroactively. Crack cocaine offenders receive longer sentences than those convicted of offenses involving powder cocaine.