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EAST GLACIER, MONTANA (WSYR) — A New York father and his 18-month-old daughter were shot and killed while on vacation in Montana, police confirmed Tuesday, saying the suspect had a prior relationship with a family member.

David Siau, 39, of Pompey, New York, and his daughter McKenzie Siau died from their injuries, while Siau’s 40-year-old wife, Christy, was in critical condition after the attack on the Blackfeet Indian Reservation in northwestern Montana.

Christy Siau was holding her daughter when the toddler was shot.

The attack happened just before 9 p.m. Sunday as the Siau family walked along a roadway in the small town of East Glacier Park.

Deputies said the suspect, 37-year-old Derick Amos Madden, of Goldsby, Oklahoma, drove his vehicle into the family, hit a tree and came out of the vehicle shooting a gun. Police didn’t immediately say how many of the family members were hit by the vehicle or how seriously injured they were by the collision.

Christina Siau, the victim’s 30-year-old sister, was stabbed by the suspect, according to investigators, who said she and the suspect had a prior relationship.

Initially, the police agency incorrectly said the prior relationship was between Christy Siau and the suspect.

Despite being seriously injured, Christina Siau defended herself and killed the shooter. Police did not say what or if a weapon was used.

Deputies say Madden “was suffering from mental health issues.”

The couple’s two other children were with them at the time of the shooting but were not hurt, according to police. They are staying with their grandparents.

The Glacier County Sheriff’s Office described it as an “isolated…incident with a clear nexus between the victims and Madden” and said the investigation was continuing. It was not clear what provoked Sunday’s attack beyond the prior relationship between Madden and Christina Siau.

Local authorities did not know of any previous interactions that Madden had with law enforcement. He had been working recently in East Glacier, a tourist town and gateway to the mountains in Glacier National Park, according to Jody Hickey, a spokesperson for Glacier County Sheriff Vernon “Napi” Billedeaux.

Christina Siau was originally from New York and had been living in East Glacier, where her family was visiting, Hickey said.

The family is active with Pompey Community Church, according to people who know them. David Siau worked for Benchmark Media, according to the couple’s wedding announcement.

In a statement to WSYR, the family’s former church called this a “time of unimaginable grief,” writing: “They were beloved & served joyfully in many ways in our church community. … When they purchased their home in Pompey and made the transition to Pompey Community Church, we celebrated with them as they found a wonderful place to live and to worship with their family.” 

A photo of the family was posted on the Facebook page of Mission Hope Community, with the caption: “We are deeply grieving the gun violence assault on Christy Brownell Siau and her family while they were camping on the Blackfeet Indian reservation. Christy was the bookkeeper for ABC-NYS for several years. Her husband David is dead, along with their 18-month old daughter, and Christy is hospitalized. G-d be with them, and G-d forgive us for allowing our idolatrous gun worship to continue.”

In a message shared on Facebook by the group “American Baptist Churches,” the local director wrote: “Please remember both the Siau and Brownell families in prayer as they grieve this incredible and sudden loss.”

The Associated Press contributed to this report.