If we could find a copy of the Grand Forks Daily Hearld dated closer to the date of discovery of the bodies there might be more accurate info. Every newspaper article shows a little different info. This article dated 28 June 1915 shows, man, woman, and kids.
http://chroniclingamerica.loc.gov/lccn/sn89074405/1915-06-28/ed-1/seq-1.pdf a local paper should have more accurate info you would think.
I looked through the 1900 census for Niagara, North Dakota, searching for any boarders who were farm laborers or housekeepers. Then I searched for those same people in the 1910 census. The females are trickier because they might have married between 1900 and 1910 and thus be "lost."
There is a family on the 1900 census that stands out as a possible clue. If a fresh set of eyes wants to look, here's the info
Page 1 of the 1900 U.S. Census for Niagara, Grand Forks, North Dakota:
James Yates, age 38, born April 1862 in Missouri, farm laborer
Martha E. Yates, age 47, born March 1853 in Indiana, servant
Ethel Yates, age 15, born Dec 1884 in Missouri, in school
Eugene Lyons, age 29, born May 1861 in New York, farm laborer
These four people were boarders at the home of John and Viola Black in 1900. John and Viola had an adopted daughter named Bessie, age 6, also living in the home. John Black was a farmer in Niagara, ND. Sometime between 1900 and1905, the three people in the Black family - John, Viola, and Bessie - had moved to Minnesota. I don't know what became of their boarders.
It looks like the Yates and the Blacks are in-laws - perhaps Martha Yates and Viola Black are sisters. ? Eugene Lyons looks like an unrelated boarder. The census indicates their occupations, but not the name of their employer so I obviously don't know that they worked for Eugene Butler. However, John Black was from New York like Eugene Butler and they were near in age. (Niagara ND is named after Niagara NY where many residents moved from). Niagara ND was a small place to begin with, and being from the same state, I'm sure Eugene Butler and John Black knew each other - and it's possible Butler hired the inlaws/boarders of his hometown acquaintance. I'm just guessing, of course!
So, FWIW, I cannot find these four boarders on the 1910 census, though that doesn't necessarily mean anything other than I can't find them. If anyone out there wants to look for them, please do so!
Is this a lead or dead end?!
ETA: Dead end.