Maricopa County sheriff's deputies who were called to the family home in Mesa earlier in the day arrested Kemp Crowley, who was standing near the front door with a gash on his head as they arrived.
"He made a couple of comments that did not appear to jibe with what we found in the house," Sheriff Joe Arpaio said. "He said he was hit in the face by the brother of the wife, but, the brother has been shot many times."
Investigators believe a running custody dispute over the couple's 11-year-old daughter might have led to the slayings.
The Arizona Republic said Wednesday neighbors had not considered Crowely to be violent despite his large physical size and often-unkempt appearance.
"He was very hard-working. He loved his family," said neighbor Elizabeth LaRusso. "He never hit his kids. He never hit her."
Other acquaintances, however, told the newspaper that Crowley's 42-year-old wife, Tammi Meininger, had said she was afraid of her estranged husband.
Meininger was found dead with the body of her 3-year-old son, Austin Potter, in her arms. Austin suffered from cerebral palsy. Also killed were Meininger's brother, Mark Potter, 40; sister Terri Borden, 45; and mother Joyce Levi, 65.
Two other children who lived in the home -- ages 11 and 12 -- were at school when the killings occurred, Arpaio said.