Seattle ? After slaughtering their parents, Joseph McEnroe apologized to his girlfriend’s young niece and nephew before shooting both in the head to end a Christmas Eve massacre, prosecutors alleged Friday.

But even as they filed aggravated first-degree murder charges against McEnroe and Michele Anderson, prosecutors could not say what might have driven the couple in the violent killing spree.

“In the end, what motive could you find that would make sense of the senseless slaying of the Anderson family?” King County Prosecutor Dan Satterberg said in announcing the charges.

Anderson and McEnroe, both 29, were each charged with six counts of aggravated first-degree murder. Authorities say they have confessed. Conviction on aggravated first-degree murder in Washington is punishable only by death or life in prison without possibility of parole, and Satterberg said he would give “serious consideration” to the death penalty.

Telephone calls to public defender George Eppler, Anderson’s attorney, and Devon Gibbs, McEnroe’s attorney, were not returned Friday.

In a Friday afternoon interview from jail with The Seattle Times, McEnroe said he regrets the deaths of his girlfriend’s family members, but he did not admit to playing any part in killing them.

“I’m sorry that they’re gone. They were my family, too, you know?” McEnroe told the newspaper. “I hope wherever they’re at, they’re at peace. That’s all I’m going to say about them.”

According to court documents, Anderson said both of them shot her parents, brother and sister-in-law, while McEnroe killed the children. While long-standing bitterness and a perceived family debt might have been factors in the killings, the motive may never be known.