After more than a year of delays, a trial challenging Tennessee's method for executing prisoners has begun.

Steve Kissinger represents 33 death row inmates who say lethal injection is unconstitutional. In opening statements, Kissinger said the state's use of prison guards to inject the drug creates a substantial risk that it will be administered incorrectly and cause extreme pain.

Assistant Attorney General Scott Sutherland countered that the U.S. Supreme Court has said inmates aren't guaranteed a painless death. Sutherland says the execution method is cruel and unusual only if it involves things such as torture or the deliberate infliction of pain.

Tennessee has yet to carry out an execution using the drug pentobarbital. But Sutherland says Texas, Ohio and Georgia have carried out more than 30 successful and painless executions with the drug.

The lawsuit is one of several challenges to various execution protocols nationwide.