Parents of US school shooter Natalie Rupnow are being investigated for negligence following the atrocity that left two people dead and six injured.

Rupnow, who went by the name Samantha, opened fire at Abundant Life Christian School on Monday morning, killing another student and a teacher, and wounding six others. Police Chief Shon Barnes said the teacher and student were pronounced dead at the school in Madison while Rupnow also died, likely by suicide.

Two students remained in critical condition Tuesday, police said. A teacher and three students were taken to a hospital with less serious injuries, and two of them had been released by Monday evening. Detectives investigating the shooting spree are looking at whether the parents of the 15-year-old, Jeff and Melissa, had any responsibility. “We also want to look at if the parents may have been negligent,” said Police Chief Barnes.

Rupnow pictured at a firing range (
Image:
Facebook)
Rupnow pictured with dogs (
Image:
Facebook)

Officers have been talking with Rupnow’s father and other family members, who are understood to be cooperating, while the shooter’s home has also been searched. Madison Mayor Satya Rhodes-Conway said that it was “far too early” to say if the parents would be charged or not.

She added: “There is so much that we do not know and we have to allow law enforcement the time and space for a careful and methodical examination.” Police believe that the shooter used a 9mm gun and they are still investigating how Rupnow got hold of it - and if it was owned by her parents.

Police at the Rupnow home (
Image:
Getty Images)

Barbara Wiers, campus director at Abundant Life Christian School, said that the school does not have metal detectors but uses other security measures including cameras. She also said guns are not allowed on campus and that the school regularly practices safety routines.

The teenage shooter came from what was reportedly an unsettled home life with her parents divorcing and remarrying several times. She had therapy, according to the Washington Post, which is understood to have taken place as she was deciding which of her parents to live with.

Students escorted from the school (
Image:
Getty Images)

Dad Jeff posted many photos on Facebook which seemed to show happy memories of him with his daughter including of Natalie returning home after she was born in 2009. Other photos included beach holidays and one of Natalie at a martial arts event.

Photos have also been shared, which were on social media, of Natalie at a firing range pointing a gun and wearing a t-shirt similar to that worn by Columbine killer Eric Harris. Mr Rupnow appeared to be encouraging her daughter to use his shotgun with the image of her aiming at clay pigeons at a local gun club.

“Is that kiddo?” a friend commented, reported the Independent. “Sure is!!!!” Mr Rupnow replied. “We joined NBSC this spring and we have been loving all [sic] every second of it!”