Three police officers failed to take “an investigative mindset” when looking into the death of a three-year-old boy that later transpired to have been a murder, a misconduct hearing has heard.

DCI Mark Swift, DI George Bardell and PC Oliver Scoones are alleged to have breached professional standards in the way they investigated the death of Riley Siswick.

Riley was found dead at his home in Huddersfield, West Yorkshire, on 6 February 2016, with Kyle Campbell eventually being convicted of murdering him, and the boy’s mother, Kayleigh Siswick, being found guilty of causing or allowing the death of her child.

The misconduct hearing heard on Monday that the officers who investigated Riley’s death had initially failed to seriously consider the possibility that it may have resulted from a “traumatic assault”.

Briefly outlining the case against the officers, Ian Skelt told a panel sitting at the West Yorkshire police headquarters in Wakefield that officers from the force’s homicide and major enquiries team should have been assigned to the case from an early stage but were not.

He told the hearing paramedics had attended Riley’s home on the day he was found dead. “It was noted that the child’s mother was in the home – she appeared to be hysterical,” he said.

Skelt said that after the child had been declared dead, a consultant paediatrician identified 14 separate bruises and marks on his back. However, he said, it was felt at the time that none of the marks helped identify a cause of death and no photographs were taken of the bruises. Photographic evidence should have been taken, the misconduct panel was told.

The hearing heard how, based partly on information provided by Bardell, the case was investigated by district officers, as opposed to West Yorkshire police’s homicide and major enquiries team.

Skelt said: “Even at these early times, there were indications that the death may well have been suspicious.”

He said occurrence enquiry logs made on the police system by Scoones a day after the death did not indicate “any apparent concern on his part that the death may be anything other than natural”.

In a postmortem examination held on 8 February 2016 in the presence of Scoones and Bardell, it was said the boy’s bowel had been separated and that this was the probable cause of death, the misconduct hearing heard.

The hearing was told that, in a log written the next day, Bardell stated that the injury seemed to be “50/50 between being a medical defect and a trauma”, with officers investigating the possibility Riley had a “slip, trip or fall”.

Skelt said: “The officers appeared to have been focused, for whatever reason, on it being an innocent cause of death, rather than taking an investigative mindset and considering the possibility this may have been a traumatic assault.”

A trial at Leeds crown court this year heard how Campbell, the then partner of Kayleigh Siswick, had inflicted a blow “either from a fist, or by jumping or stamping” on Riley on 4 February 2016. He was jailed for life with a minimum of 20 years while Kayleigh Siswick was given a seven-year sentence after it was heard her son had been left to suffer for two days.

The three officers are accused of failing to conduct a thorough investigation and preparing a report to a coroner that was incomplete or misleading.

Bardell and Swift are accused of failing to supervise an investigation appropriately, with Scoones alleged to have attempted to influence a witness over the evidence they provided.

The hearing, which is due to last two weeks, continues.