PORTLAND – Maine’s highest court Thursday denied the appeal of a Waterville man found guilty of hatching a murder-for-hire plot targeted at a woman who was prepared to testify against him in a firearms case.

Mathieu O’Rourke, 23, sought to set aside last year’s conviction of solicitation to commit murder by challenging the trial judge’s rulings on a host of evidentiary issues including the admission of coded letters found in his jail locker.

The Supreme Judicial Court ruled unanimously against O’Rourke on all points raised in his appeal.

O’Rourke was sentenced last year to eight years in prison and six years probation.

The intended victim of the plot was a 26-year-old female co-worker of O’Rourke who authorities said was prepared to testify that she saw him playing with her handgun in her Waterville apartment and that the weapon later turned up missing. The woman was not harmed.

Because he was a convicted felon who was on probation, O’Rourke could have faced a five-year sentence if found guilty of being in possession of a firearm, the prosecution said at last year’s trial in Kennebec County Superior Court.

O’Rourke allegedly tried to hire Ramon Davila, a former inmate whom he met at the Kennebec County Jail, to kill the woman in return for $10,000. O’Rourke sent letters written in code to Davila, who responded that he completed the job even though he never intended to kill anyone.

When O’Rourke realized the witness was still alive, he approached another inmate, Anthony Dorothy, who was awaiting sentencing on federal charges.

Dorothy agreed to help O’Rourke get in touch with a “hit man.” But hoping it might get him a shorter sentence, Dorothy told authorities about the plot and put O’Rourke in touch with a federal agent posing as the purported killer.

The high court upheld the admissions of conversations between O’Rourke and the undercover agent, rejecting arguments that they were obtained deceitfully. The opinion noted that Dorothy was not placed in O’Rourke’s cellblock as an informant but had been assigned there before authorities ever got wind of the plot.

The opinion also concluded that the search of O’Rourke’s jail locker without a warrant did not violate his Fourth Amendment protections.

Rejecting his claim that he had a reasonable expectation of privacy, the court noted that jail officials could examine the locker and take an inventory of its contents whenever items were added or removed.