The death penalty is under what may be an unprecedented review in the United States, mostly involving questions about lethal injection, by far the most common method of execution.

About one-third of the 38 states that allow capital punishment have halted or delayed executions while legal and ethical challenges are resolved.

The latest was Tennessee where the governor recently ordered a 90-day halt to executions, including two that were scheduled for February. He called for a comprehensive review of current execution procedures and future protocols.

Governor Phil Bredesen said he supports capital punishment, but the state, in preparing to address a legal challenge to lethal injection, had found "deficiencies with our written procedures that raise concerns that they are not adequate to preclude mistakes in the future."

One issue in Tennessee involves vagueness on how much of the deadly drugs need to be administered, he said. Nationwide, some state lawmakers and courts are debating whether the condemned unduly suffer during execution, in violation of the Constitution's guarantee against cruel and unusual punishment.

Last year, Florida halted executions after a convicted killer took 34 minutes to die when the needles carrying the drugs were improperly inserted.