YAKIMA, Wash. — A couple accused of stalking and ambushing Yakima real estate broker Vern Holbrook while posing as homebuyers were arraigned Tuesday in Yakima County Superior Court. Meanwhile, investigators are trying to determine a motive behind the attack.

Luis Gomez-Monges, 38, and Adriana Mendez, 23, entered not guilty pleas to charges of attempted first-degree murder and first-degree assault in the attack on Holbrook, who investigators say remains hospitalized.

The pair were arrested May 27, two days after a colleague of Holbrook’s found the 78-year-old real estate broker in a pool of blood at a vacant home for sale in the Cowiche area. He had been severely beaten and his throat was cut.

According to arrest affidavits, Mendez told detectives she and Gomez met Holbrook at the home by posing as prospective buyers and had asked for him specifically after calling his agency, Aspen Real Estate, days earlier.

Deputies said Mendez told them she saw Gomez punch Holbrook in the head before she left the bedroom where the attack took place and then waited in a car outside with the couple’s three children.

Detectives used cellphone records from Holbrook’s missing iPhone to track down the suspects, who were living at a motel in Yakima.

Holbrook was taken to a Seattle hospital, but no information on his condition is being released. Chief of Detectives Stew Graham confirmed Tuesday that Holbrook remains hospitalized.

A woman at Aspen Real Estate who did identify herself said the business would have no comment to honor the family’s request for privacy.

A motive for the attack remains unclear. When pressed, Graham said he and his colleagues were aware of a number of the rumors, including one that Holbrook was the victim of “the Russian mafia.” He did not elaborate on the rumor.

Nevertheless, he suggested an obvious motive remains elusive. Gomez’s court-appointed attorney, Paul Kelly, said he, too, is operating in the dark, and suggested the material he’s been given so far is so scant that he scheduled a hearing right away for next week to make sure prosecutors are turning over everything they’ve got.

“There’s a lot going on here,” he said.

The defendants remain in custody at the county jail in lieu of $750,000 bail each. Gomez is also being held on a no-bail immigration detainer.