Sept 26 (Reuters) - Alabama is planning to execute

convicted murderer Alan Miller on Thursday in the second ever

nitrogen-asphyxiation execution since the state pioneered the

method in January.

Miller, 65, was convicted for the 1999 murders of three men,

including two co-workers, in a shooting spree at two offices in

Pelham, Alabama.

In January, Alabama executed Kenneth Smith, also convicted

of murder, by nitrogen asphyxiation, the first use of a new

execution method since lethal injections began in the U.S. four

decades ago.

The state called its new protocol "the most painless"

execution method yet, and predicted that Smith would lose

consciousness and suffocate within a few seconds of pure

nitrogen flowing through an industrial-safety respirator mask

strapped to his face.

That did not happen. Multiple witnesses, including five

journalists and members of Smith's family, saw Smith heaving

against his restraints and convulsing as the nitrogen flowed,

and it appeared to take him several minutes to lose

consciousness.

The administration of U.S. President Joe Biden, a Democrat

who promised to abolish the death penalty, said it was "deeply

troubled" by the new method. It has been condemned by senior

U.N. human rights officials, who say it may amount to torture or

cruel and inhuman treatment.

Alabama has defended the method, with Attorney General Steve

Marshall calling Smith's death a "textbook" execution.

After Smith's execution, Miller sued Alabama over fears its

asphyxiation method could violate a constitutional ban on cruel

and unusual punishments. He sought assurances that the mask

would be properly fitted and that its seal be tested to ensure

no oxygen seeps in, prolonging his death or even averting it

entirely and leaving him alive with brain damage.

His litigation ended in a confidential settlement, and his

execution is scheduled for 6 p.m. at the Holman Correctional

Facility in Atmore, Alabama. The Alabama Department of

Corrections has declined to say if it made any changes to its

gassing protocol.

Alabama has offered to help others adopt

nitrogen-asphyxiation executions, saying they are a simpler

alternative for states struggling to find lethal-injection

drugs.

Advocacy groups against capital punishment have long

pressured pharmaceutical companies to forbid their drugs be used

in executions, and have turned their attention to nitrogen

suppliers and mask makers, including the maker of the Allegro

Safety mask Alabama acquired for its executions.

Stephanie Boucher, a spokesperson for Allegro's parent

company Walter Surface Technologies in Connecticut, said the

company will not comment on the use of its masks in executions.

A spokesperson for Canadian private equity firm Onex Corp ( ONEXF )

, which has a majority stake in Walter Surface, did not

respond to requests for comment.