Ivory Warren, one of the two men accused of killing a Gretna cab driver two years ago, is asking a judge to change his plea of not guilty to one of not guilty by reason of insanity, with his attorney writing that his client “has a documented history of mental illness and intellectual disability.”

Warren, 20, and Jonah Brown, 19, were accused of shooting Blake Helmer after hailing Helmer's cab on April 29, 2014. Police said the pair got into the vehicle near Hancock and Rupp streets in Gretna, demanded money from Helmer and then shot him after he resisted.

Helmer, 55, called a cab dispatcher on his radio about 8 p.m. and said he had been shot four times. He was found by responding officers on the ground in the 900 block of Rupp Street, alive but wounded.

According to police testimony at a December court hearing, Helmer told the officers he had tried to fight back during the robbery. He died later at a hospital.

Brown and Warren, who will be tried separately before Judge Stephen Enright in 24th Judicial District Court in Gretna, both face charges of second-degree murder, along with conspiracy to commit armed robbery. Brown also is charged with conspiracy to obstruct justice.

The motion to change Warren’s plea does not include any specifics about his mental condition, although attorney John Thomas wrote that there is “substantial evidence” that Warren “was incompetent/insane at the time of the offense.”

Thomas also claimed that Warren’s youth at the time — he had turned 18 the same day — “rendered him incapable of forming specific intent to commit murder.”

Thomas could not be reached for comment Tuesday, though he indicated while questioning a deputy in December that Warren had been taking prescription medication for depression at the time he was arrested.

Earlier this year, Warren tried unsuccessfully to argue that he was born 57 minutes too late to be charged with second-degree murder.

In a motion to quash the indictment filed in March, Thomas pointed out the police report says the shooting happened at 8:08 p.m. on Warren’s 18th birthday, while his birth certificate shows he was born at 9:05 p.m.

That would make Warren 17 years old at the time of the offense, Thomas argued, rendering him ineligible to receive the mandatory sentence that comes with a conviction for second-degree murder: life in prison without the possibility of parole.

The state Attorney General’s Office, however, argued that the wording of the state law crafted in 2013 in response to a U.S. Supreme Court decision outlawing automatic life sentences for juveniles would in fact allow Warren to get less than a life sentence if he were to be convicted as charged.

In a separate filing, prosecutors with District Attorney Paul Connick’s office said higher courts have ruled that, for practical reasons, fractions of days are not used in calculating ages in such matters.

Enright denied Warren’s motion in April.

Follow Chad Calder on Twitter, @Chad_Calder.