Police are investigating what led to the deaths of a 64-year-old grandmother and three of her grandchildren on a North Dakota reservation. Reports state that a "person of interest" killed himself hours after the four murders.

Martha Johnson, 64, and three of her grandchildren: Benjamin Schuster, 13; Julia Schuster, 10; and Luke Schuster, 6, were all shot while at their home on Sunday afternoon. Another child was in the home but managed to avoid injury and was able to call 911.

While authorities will not release the name of the person of interest at this time, they have confirmed that he was a registered member of the American Indian tribe on Fort Berthold. He killed himself with a knife when confronted by police, Mountrail County Sheriff Ken Halvorson told the Associated Press.

An investigation is still pending as authorities try to figure out exactly what happened on Sunday. The surviving child may be able to help police with their investigation, providing crucial details as to what transpired in the home.

The Fort Berthold community is normally quiet, with fewer than 2,000 residents, and friends described Johnson as "a nice lady. She was always out in the yard working. It's very sad. It's unreal," said neighbor Megan Hale.

It's a shock to the community, which canceled school in order to allow students to grieve and stay with their parents. The Johnson's home was only 150 yards from the local elementary school, adding a permanent reminder of the tragedy. Residents organized a prayer vigil to help bring a sense of peace to the community.

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"It's a terrible loss, especially when young kids are involved. It's a sad, sad day," said Tex Hall, the Three Affiliated Tribes chairman said.

The Three Affiliated Tribes are comprised of Mandan, Hidatsa and Arikara members who live in Fort Berthold. It was created in 1870 and has housed American Indians ever since.