SANDPOINT — More irons are being added to the fire in a bid to remove the death penalty from the first-degree murder case against Jacob Corban Coleman, according to 1st District Court records.

Counsel for Coleman filed a 91-page motion Thursday arguing that he should not pay the ultimate penalty for stabbing a Spokane Valley cab driver to death in Kootenai due to his age. Coleman was 19 when he killed Gagandeep Singh inside his minivan taxicab.

Coleman’s capital defense counsel, Eagle attorney R. Keith Roark, contends that there is growing acceptance of scientific research here and abroad which holds that human brains aren’t fully developed until at least the age of 21 and the 8th Amendment of the U.S. Constitution guarantees someone of Coleman’s age at the time of the crime cannot be executed or made to serve a life sentence without possibility of parole.

Roark points out in the motion to strike the death penalty that there is a national consensus which reflects that offenders under the age of 21 should not be put to death. Twenty-three states, in addition to the District of Columbia and five U.S. territories, don’t execute offenders who are below that age. Roark said that position is rooted in scientific research involving the role and development in the brain’s prefrontal cortex, the area of the brain which directs cognitive behavior, decision making and personality.

The research holds that it’s the last part of the brain to fully develop and undergoes major neural reconstruction through the early 20s, according to Roark. That research is supported by U.S. laws and mores which are pegged around the age of 21 — alcohol consumption, gun ownership in some states and car rentals.

“The age of 18 is not, and has never been, a true age of maturity and adulthood. It was chosen for expedience. There is no principled reason to treat those who are still immature as if they are fully developed adults,” Roark said in the motion.

The motion follows other motions to strike the death penalty. Those motions argued that Coleman should not be put to death by lethal injection because Idaho’s methods would amount to cruel and unusual punishment and because Coleman was never indicted by a grand jury for Singh’s killing.

Coleman, now 20, is being held at the Bonner County Jail. His trial is scheduled for this spring.

Keith Kinnaird can be reached by email at kkinnaird@bonnercountydailybee.com and follow him on Twitter @KeithDailyBee.