Oklahoma could resume executions as early as the end of the year, becoming the first state ever to use nitrogen gas.

"This is the safest, the best and the most effective method and we are moving forward," Attorney General Mike Hunter said at a news conference Wednesday announcing the change.

Opponents of the death penalty quickly condemned the decision.

"This method has never been used before and is experimental," said Dale Baich, an assistant federal public defender and one of the attorneys representing 20 death row inmates in a federal lawsuit. "How can we trust Oklahoma to get this right when the state's recent history reveals a culture of carelessness and mistakes in executions?"