FOND DU LAC, Wis. --

One Fond du Lac police officer was killed and another was critically injured when a man inside a home on the city's west side began firing.

Police Chief Tony Barthuly said 28-year-old Officer Craig Birkholz was shot in the upper chest by the suspect and killed. Officer Ryan Williams was shot twice in the chest and is in critical condition at a Neenah hospital.

Barthuly said Williams likely would have died had he not been wearing a protective vest.

"It's very sad that we lost an officer. It's my biggest nightmare and I think it's the biggest nightmare of any police officer or parent or family member of a police officer," Barthuly said.

Another officer, Zach Schultz, was injured and treated at the scene.

Authorities went to the house around 6 a.m. Sunday to investigate a reported sexual assault at the home. Investigators said when officers arrived, they were fired upon by a man inside.

That man, 30-year-old James Cruckson, died from a self-inflicted gunshot wound. He was found inside the house after a nearly 6-hour standoff.

Barthuly said Birkholz was on the police force for two years and also spent 5 years in the Army where he served in Iraq and Afghanistan. Birkholz is originally from Kenosha and went to UW-Oshkosh where he got his degree in criminal justice. He is survived by his wife and parents.

A K9 was also shot, but is expected to survive.

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