Updated

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 lawyer, 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.

Click here to read the exclusive Seattle Times interview

"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."

He said jailers had placed him on a suicide watch.

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.

Court documents said McEnroe, a store clerk, and Anderson, who is unemployed, told detectives they armed themselves on Christmas Eve and went to her parents' home near Carnation, about 25 miles east of Seattle. There, they confronted Anderson's parents, Wayne Anderson, 60, and Judy Anderson, 61, in their living room.

Michele Anderson told detectives her brother, a carpenter, owed her money she had loaned to him years earlier, and that she was upset with her parents because they did not take her side, documents say. She also said her parents were pressuring her to start paying rent for staying on their property, where she lived in a trailer with McEnroe.

"Michelle stated that she was tired of everybody stepping on her," the court papers say. "She stated that she was upset with her parents and her brother and that if the problems did not get resolved on Dec. 24, then her intent was definitely to kill everybody."

Satterberg said Michele Anderson fired once at her father's head but missed. McEnroe stepped in, leveled his gun and fatally shot Wayne Anderson in the head, documents said.

Judy Anderson heard the shots and ran from the back room where she had been wrapping gifts. She was shot by McEnroe, who apologized to her before shooting her again, this time in the head, the court documents said.

Satterberg said that the two dragged the bodies to a shed behind the house, used towels and carpets to sop up blood stains and awaited the arrival of Anderson's brother, Scott. He was due for a Christmas Eve visit with his wife, Erica, and children Olivia, 6, and Nathan, 3.

Her brother and sister-in-law put up a brave struggle, according to the documents. Scott Anderson charged her when she pulled out the gun, and was shot up to four times, records say.

Michele then shot Erica Anderson twice, but she was able to crawl over the back of a couch to call 911, authorities said. McEnroe told detectives he tore the phone from Erica's hands and destroyed it.

Huddling with her children, Erica Anderson pleaded with McEnroe not to shoot her, saying: "You don't have to do this."

"Yes, we do," McEnroe was quoted as replying in the affidavit. He fired at her head, authorities said.

Satterberg said that McEnroe apologized to both of the children before he shot 6-year-old Olivia. He then turned to 3-year-old Nathan, who had picked up the batteries from the cordless phone his mother had used in her futile attempt to call for help, court documents said.

McEnroe than fired one last bullet through Nathan's head, according to the affidavit. When asked why he shot Erica, Olivia and Nathan, McEnroe told detectives three times: "I didn't want them to turn us in."

After the killings, McEnroe and Anderson first drove north toward Canada, then south toward Oregon arriving at neither destination, then decided to go back and pretend to discover the bodies, Satterberg said.

When they arrived Wednesday, investigators were already there. Detectives, curious that neither McEnroe nor Michele Anderson asked what had happened at the bustling crime scene, began questioning them and they eventually confessed, according to the documents.