A Somali man charged in the fatal shooting of a 19-year-old woman in northwest North Dakota is now accused of killing her mother, brother and the mother's boyfriend, who were found dead the same day in mobile home across town.

Omar Mohamed Kalmio, 27, who has a history of violent crime, was charged with murder several months after his infant daughter's mother, Sabrina Zephier, was found dead at her home in Minot in January 2011. The baby was found in the home unharmed.

Court documents show prosecutors filed additional murder charges Friday, alleging Kalmio also was the gunman who killed 13-year-old Dylan Zephier, 38-year-old Jolene Zephier, and Jolene's 22-year-old boyfriend, Jeremy Longie.

Multiple slayings are virtually unheard of in North Dakota, which had 10 murders and non-negligent homicides in all of 2010, according to data compiled by the FBI.

Ward County State's Attorney Roza Larson declined to comment on specifics of the case Wednesday, but she cited the ongoing work by Minot police and the state Bureau of Criminal Investigation. Police also declined comment.

"They have been working this case since the murders occurred and they have been putting in a lot of tireless hours," she said. "A lot of times when things take a while, people have the perception that investigators aren't doing anything. They truly have been doing a lot."

Kalmio, who has pleaded not guilty in Sabrina Zephier's death, is being held without bond. His trial was scheduled to begin June 4, but was cancelled when the new charges were filed. His court-appointed attorney couldn't be reached for comment Wednesday; a recorded phone message said his mailbox was full.

Kalmio was arrested last August at the Grand Forks County jail, where he had been held since February 2011 on an unrelated charge. Police have said that last summer's devastating flooding in Minot may have delayed his arrest, because police had to divert their efforts elsewhere.

The Somali national has said he was in the U.S. under political asylum. He's scheduled for a preliminary hearing June 19, and Larson said she expects the case to go to trial.

Sabrina Zephier's body was found in an apartment on Jan. 28, 2011, the same day the bodies of the other three were found in a nearby mobile home. They all had been shot.

The Zephiers were members of South Dakota's Yankton Sioux Tribe.

Police contend in court documents that a witness claimed Kalmio and Zephier argued days before she was found dead, and Kalmio's co-workers at an oil rig site near Williston claimed Kalmio had made statements about Zephier purposely getting pregnant and ruining his life. Kalmio also allegedly described injuries to Zephier that had not been publicly disclosed, authorities said.

Kalmio was convicted in 2006 of second-degree assault with a dangerous weapon and sentenced to about a year in prison. He and a group of other Somali men were accused of attacking a man in Minneapolis in January 2006, and Kalmio stabbed him three times in the back with a knife. Kalmio also was convicted of theft in 2006, and ordered to pay a fine.

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Associated Press writer Blake Nicholson in Bismarck contributed to this story.