LITTLE ROCK, Ark. - Arkansas plans to put prisoners to death with a drug that apparently hasn't been used in a U.S. execution, and lawyers for condemned inmates warn that it could take longer for someone to die from it than from other lethal-injection drugs.

Arkansas Department of Correction spokeswoman Shea Wilson told The Associated Press Tuesday that the state plans to use phenobarbital with lorazepam in lethal injections. Arkansas recently passed a law that will allow the state to resume executions.

Both phenobarbital and lorazepam are used to relieve anxiety. Phenobarbital, which is a barbiturate or a kind of depressant drug, is also used to control seizures.

In a letter obtained by the AP, federal public defender Jenniffer Horan told Democratic Gov. Mike Beebe that phenobarbital takes effect more slowly than other execution drugs and that it carries a "substantial risk of a lingering and inhumane death."

"No state has ever used Phenobarbital in a lethal injection procedure, and for good reason," wrote Horan, whose office represents a number of Arkansas death-row inmates. "Throughout the history of lethal injection, states have preferred ultra-short-acting barbiturates that cause rapid unconsciousness and death. Phenobarbital, however, has a long onset of action."

Richard Dieter, executive director of the Washington-based Death Penalty Information Center, also said states have never used phenobarbital in executions.

A manufacturer of propofol, the anesthetic blamed for Michael Jackson's death, said last year that it wouldn't sell the drug for use in U.S. executions, causing a setback for Missouri and other states looking for an alternative after other drugmakers also objected to their products' use in lethal injections.

"There is some uncertainty out there even in obtaining some of those drugs, so I think states are … looking into alternatives that might be more readily compounded or available," Dieter said.

Arkansas' last execution was in 2005. The state is seeking to resume executions after legislators this year rewrote a lethal-injection law that the state Supreme Court struck down in 2012.

The court ruled that Arkansas legislators had given the Department of Correction too much control over execution procedures. The new law says the state must use a lethal dose of a barbiturate, but it leaves it up to the Department of Correction to determine which drug.

Until Tuesday, corrections officials hadn't said publicly which drug or drugs they plan to use.