UNION GAP, Wash. — A Union Gap woman had her throat slashed by her boyfriend’s brother, who then slashed his brother in the face when he tried to intervene, police said.

The motive behind the attack Sunday morning remained unclear to investigators, even though the suspect was in custody, Union Gap Interim Police Chief Greg Cobb said.

“We have absolutely no idea what would have caused this,” Cobb said. “He (the suspect) hasn’t necessarily clammed up. ... We just have no idea at this point.”

The attack was reported at 10:40 a.m. when police were told a woman was being taken by private car to a hospital in Yakima.

Minutes later, personnel at Yakima Regional Medical and Cardiac Center reported that a woman and a man had been dropped off at the hospital and that both had serious knife wounds to the throat.

By then, a 35-year-old man whom Cobb described as well known to local law enforcement was identified as the suspect. The victims were his brother and his brother’s girlfriend.

Cobb said investigators determined the attack took place at a home in the 2100 block of South Fourth Avenue shared by the suspect and the victims.

He said there had been “some sort of domestic incident” between the suspect and the woman, and that the suspect slashed the woman’s throat.

The suspect’s brother heard the commotion from another room and came to his girlfriend’s aid, at which point he was slashed multiple times in the face, Cobb said.

The suspect was taken into custody while he was on foot in the 800 block of West Valley Mall Boulevard after he called 911 on his cellphone and threatened to commit suicide, Cobb said.

His brother and the woman suffered serious wounds and were in surgery hours after the attack, Cobb said, confirming reports the girlfriend’s throat had been cut from ear to ear.

The suspect was lodged in the Yakima County jail without bail pending a preliminary appearance today before a judge.

Names of the suspect and victims are being withheld pending further investigation. An update on their medical condition was not available late Sunday.

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