A man serving probation in Wisconsin for inflicting severe brain damage on a toddler is now charged with murder in Washington County in connection with the beating of a 16-month-old boy this spring.

Jonathon D. Wierstad, 28, of Willernie, had a charge of first-degree assault upgraded Monday to second-degree murder following the death Saturday of Blake Holmquist, his fiancée’s child, stemming from injuries inflicted May 11 at the defendant’s home.

Wierstad remains jailed in lieu of $2 million bail and is due back in court on June 22. His attorney declined Tuesday to speak about the allegations.

After a similarly violent incident, Wierstad and prosecutors in neighboring St. Croix County, Wis., agreed five months ago to allow him to plead no contest to causing permanent brain damage to the 3-year-old son of Stacie Nanoff, a Hudson-area woman who was his girlfriend at the time of the 2014 incident.

He spent six days in jail in January 2017 for the injuries he inflicted upon Cedarius Nanoff and was put on probation for one year.

St. Croix County District Attorney Michael Nieskes told the Star Tribune on Tuesday that his office agreed to the plea deal because “the facts in the case gave us some proof issues, and we made the decision that we thought was in the best interests of the community and in accordance with the wishes of the victim’s mother.”

Nieskes added that several other factors weighed into Judge Eric Lundell’s approval of the plea that kept Wierstad out of prison, among them that Wierstad had no criminal history up to that point and that he caused no problems while out on bond for the 2¾ years from the time of the 3-year-old’s injuries until sentencing.

The prosecutor in the Washington County case, Kevin Mueller, said Tuesday that the manner in which the injuries were inflicted on both children “seems to be very similar. That is very concerning.”

On May 11, police and paramedics arrived at Wierstad’s home, where they found the boy unresponsive, according the criminal complaint. An ambulance took Blake to a hospital, where he underwent emergency surgery for extreme swelling and bleeding on his brain.

Wierstad told police that his fiancée, Alycia Holmquist, went to pick up her 7-year-old son from school, leaving him to look after her 3- and 5-year-old children and the 16-month-old.

He said the 16-month-old fell down the stairs.

The surgeon contradicted Wierstad’s contention and said the boy’s injuries were “more akin to the type of head injury a child would suffer in a high-velocity car accident ... or severe shaking.”