YAKIMA, Wash. — Turning a West Valley triple-murder case on its head yet again, prosecutors want Kevin Harper’s guilty plea to theft and firearm charges withdrawn. And they want to explain why only in a closed-door hearing.

During a hearing Friday, prosecutors accused Harper of violating a confidential cooperation agreement behind a plea deal hammered out last October. Under that deal, Harper pleaded guilty to theft and firearms charges in exchange for prosecutors agreeing to drop three murder charges against him when he is sentenced on the other charges.

A sentencing date has never been set, however, and a close reading of the plea deal says dismissal of the murder charges against Harper was predicated upon an “agreement of the parties” that is not included in the court file.

At Friday’s hearing in Yakima County Superior Court, prosecutors did not specify how Harper, 30, violated that agreement.

Prosecutors are seeking a closed-door hearing to avoid tainting the jury pool and prompting an expensive change of venue, Chief Criminal Deputy Prosecutor Ken Ramm said when asked by the judge.

Despite a shared distaste for a change of venue, Superior Court Judge Ruth Reukauf nevertheless said she was very concerned about closing the court, which appellate courts generally frown on in criminal cases.

Instead, she ordered a June 13 hearing to give news media and other interested parties the opportunity to weigh in on the proposed closure and sealing of motions. “It’s an interesting turn in this case,” she observed.

The request to revoke the agreement drew a strenuous objection from defense attorney Pete Mazzone, who accused prosecutors of cutting the plea deal in bad faith at a time when the case against his client was in danger of collapsing last fall.

The plea deal was used to buy time for detectives to further investigate Harper, said Mazzone, who noted prosecutors turned over 1,800 pages of new evidence in the case Thursday.

Mazzone complained sheriff’s detectives got help on their second round of investigating Harper from the Yakima Police Department, Seattle police, the State Patrol and even the Attorney General’s office. He did not elaborate on their findings.

“It’s a whole new investigation, is my understanding of it,” he said.

Harper was arrested following the February 2011 slayings of Bill Goggin, owner of a Yakima engineering firm; his wife, Pauline; and his 98-year-old mother, Bettye.

They were found bludgeoned to death in their home in the gated Falcon Ridge community, west of Yakima. Investigators theorize they were killed during a burglary.

Amid a series of missteps that threatened to derail the case, prosecutors hastily cut a deal with Harper last October after a defense investigator discovered an alibi witness whose report of suspicious activity at the Goggin home was either dismissed or mislaid by investigators.

Harper subsequently pleaded guilty to two relatively minor charges — first-degree unlawful possession of a firearm and second-degree possession of stolen property — related to the theft of a Western-style .22-caliber pistol from the Goggin home.

In exchange for his plea, prosecutors agreed to recommend a prison sentence of just over seven years.

As part of the plea deal, prosecutors dismissed accessory charges against Harper’s wife, Crystal Gray-West.

A stolen property charge remains pending against Tennance Buckingham. His next court date is an evidence suppression hearing set for May 8.

A murder case against a fourth defendant, Tracy Culton, ended in February after prosecutors said the renewed investigation found insufficient evidence to proceed against her.

• Chris Bristol can be reached at 509-577-7748 or cbristol@yakimaherald.com. Follow him on Twitter at twitter.com/ChrisJBristol.