Puerto Rico's constitution expressly forbids capital punishment, stating "The death penalty shall not exist", setting it apart from all U.S. states and territories other than Michigan, which also has a constitutional prohibition (eleven other states and the District of Columbia have abolished capital punishment through statutory law). However, capital punishment is still applicable to offenses committed in Puerto Rico, if they fall under the jurisdiction of the federal government, though federal death penalty prosecutions there have generated significant controversy.
Is there an answer to this question (If it cannot be answered, say "unanswerable"): What US state prohibits the death penalty in its constitution?
Michigan