A Wisconsin professor faces serious charges after allegedly murdering his pregnant mistress and her dog, then setting her home on fire in a bid to conceal the crime. The shocking incident unfolded when the woman refused to have an abortion, according to authorities.

Matthew Sierra, a 38-year-old mechanical design instructor at Western Technical College, was taken into custody on December 16. This arrest came more than a month after the tragic death of 27-year-old Alexis Pickett, who was found in her La Crosse residence. Police reports indicate that Sierra attempted to hide his actions by igniting the apartment while Pickett’s body remained inside, as detailed in a complaint acquired by People magazine.

Investigators revealed that Sierra and Pickett already shared a child, and when Pickett became pregnant again, Sierra allegedly pressured her to have an abortion. He reportedly threatened to fight for custody of their existing child if she did not comply with his demands.

La Crosse Police Chief Shaw Kudron highlighted the extensive efforts involved in the investigation, which was carried out in collaboration with the La Crosse Fire Department, the Wisconsin Department of Justice’s Criminal Investigation division, and the Wisconsin State Fire Marshal’s Office. “Our thoughts continue to be with the Pickett family,” Kudron expressed in his statement.

“Our thoughts continue to be with the Pickett family.”

The alleged cheating killer’s bust came after firefighters responded to the blaze at the victim’s Taylor Street apartment on Nov. 13 and found her and her dog dead inside, according to La Crosse police.

An autopsy later revealed Pickett had died before the fire even started, the complaint said.

Surveillance footage captured Sierra entering his secret lover’s home at 12:45 a.m. and leaving two hours later, while smoke alarms blared and he scrambled to get into his car and flee the scene, police said.

The couple’s two-year-old son was reportedly staying with Pickett’s friend the night she was murdered.

“All I know is that they have been on and off,” the friend told police, the outlet reported.

“They have been constantly fighting because he found out she was pregnant with his kid and he didn’t want anything to do with it.”

The twisted suspect’s wife, who was aware of his affair and love child, told authorities she thought he was home the night of the killings and never saw him leave — despite cell phone data allegedly placing him at the grisly scene, the court docs showed.

Sierra, who was placed on administrative leave from his teaching job the day after his arrest, was charged with first-degree intentional homicide, first-degree intentional homicide of an unborn child, arson, and mistreatment of animals causing death.

During a preliminary hearing on Tuesday, a judge granted the defense’s motion to dismiss the professor’s charges, citing issues with the complaint, and gave prosecutors 72 hours to refile, WKBT reported.

Sierra is currently behind held on $1 million cash bond a La Crosse County Jail.

He is due back in court on Friday afternoon.