A US police officer murdered his British wife, their two children and his mother-in-law before killing himself after his wife found out that she had been repeatedly drugged and raped by him, a police report has found.

The Spanish Fork Police Department in Utah have only just finished their investigation into Joshua Boren’s shootings in January.

He went on the rampage in the family home, using his police gun to shoot dead his seven-year-old son, Joshua, and five-year-old daughter, Haley.

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Officers found that 32-year-old Kelly Boren, who was originally from Northampton, had been drugged and raped by her husband several times over a period of time.

Boren's therapist told authorities the police officer filmed the assaults and his wife only found out what had happened when she discovered the tapes in 2013.

She told a few friends but did not report it to police because she did not want to ruin her husband's career, the report said.

The couple had reportedly been separated for some time when Mrs Boren confronted her husband about the rapes by text, the night before she was killed.

A Spanish Fork Police report shows Boren, 34, and his wife exchanged heated texts the night and morning before the January killings.

The Deseret News reported that she texted her husband saying their marriage was over.

She texted the word "rape" to him four times, the documents show.

"I hate my life because (of) you," she texted. "You killed a part of me."

She wrote in another text: "I don't want to live in fear and hate and anger."

The next morning, Mrs Boren told her husband she would take the children. He replied: "Don't involve the kids, they are innocent."

According to the police report, Boren was sexually abused as a child, struggled with drug addiction as a young man and pornography addiction throughout his life, and had a deep-rooted hatred for his mother.

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After his father committed suicide when he was five, she began using drugs and seeing several men, one of whom allegedly abused Boren.

His sister told police he blamed his mother for not protecting him and his therapist described him as a “three-year-old boy stuck in a big man’s body”.

"Josh was a very troubled individual that felt like he was about to lose his wife and children," police wrote in the report.

He had worked for the Lindon Police Department for only three months before the murder-suicide.

Before that, he was a Utah County sheriff's deputy for seven years, said Matt Johnson, from Spanish Fork Police.

He used the police handgun he was given for his duties to murder his family and mother-in-law, Mary King.

Toxicology reports from the autopsy showed he had no drugs or alcohol in his system when he carried out the shooting.

It shocked the city of Spanish Fork in January, as well as friends and family, and investigators said they did not find anyone who believed Boren was capable of the murder.

Despite not living at the house, he reportedly arrived every morning to get the children ready for school and picked them up in the afternoon.

"He was praised as being an excellent father," Mr Johnson said.