A 29-year-old man has been arrested on suspicion of fatally beating a 7-month-old boy last weekend at the central Minnesota home of his girlfriend, authorities said Tuesday.

Late Saturday morning, the infant’s 33-year-old mother called 911 from her home southeast of Rice and said she believed her son was dead, according to the Benton County Sheriff’s Office.

Sheriff’s deputies and other emergency responders arrived at the home in the 1100 block of 108th Street NW. in Watab Township, located the boy and began lifesaving efforts, the Sheriff’s Office said.

However, the baby was declared dead at the scene. The Midwest Medical Examiner’s Office in Anoka ruled that he died from “blunt force trauma.”

Law enforcement has yet to reveal a motive for the beating, and it has not identified the infant or his family.

The 29-year-old suspect, from St. Joseph, Minn., “was in a significant relationship with the infant’s mother” and had arrived at her home Friday night with plans to stay the weekend, the Sheriff’s Office said.

Earlier Saturday morning, before the boy’s mother had called 911, her boyfriend was arrested near Royalton on drunken-driving and assault charges. While in custody in the Morrison County jail, he refused a preliminary breath test, physically resisted being led into his cell and punched a sheriff’s deputy, the charges read.

The suspect was moved to the Benton County jail Tuesday afternoon. Charges in the baby’s death are pending. The Star Tribune generally does not identify suspects before they are charged.