CBS affiliate KOTV spoke to Larry Powell, the father of the twins, who confirmed that the 3-year-old was his grandson.
"The little baby, you know, he had to sit there all day and look at what they did. How could you do something like that?" Powell said, according to the station.
KOTV reports this is not the first time Powell has experienced the pain of losing a child. His son died from the flu in 2003. Now, his two remaining daughters are also gone.
Officers don't know what relationship the other victims, women ages 33 and 55, had to the twins or the child. Tulsa police spokesman Leland Ashley said Tuesday there is no "clear-cut suspect" and that investigators don't know why the women were killed.
The Fairmont Terrace apartment complex has a nighttime security patrol but police believe the killings occurred in broad daylight.
The bodies were found midday Monday, about an hour after anyone last heard from someone at the apartment.
The neighborhood is about a mile from the Southern Hills Country Club and two miles from Oral Roberts University.
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