A police sergeant was arrested Thursday on suspicion of killing his wife found dead Tuesday in Fukuoka Prefecture.

Mitsuru Nakata, a 38-year-old sergeant with the Fukuoka Prefectural Police, is accused of being involved in the deaths of his family, including 38-year-old wife Yukiko Nakata, son Ryosuke, 9, and daughter Miyu, 6.

Their bodies were found at their home and Nakata denies the allegation.

The sergeant works for the prefectural police’s communications dispatch division.

“This is an unprecedented scandal in the history of the prefectural police,” said an investigative source.

The police initially suspected murder-suicide, given the signs of strangulation on the two children. However, the police now suspect Nakata tried to alter the scene to point to suicide.

An autopsy later found signs the wife had also been strangled, prompting the police to treat the case as a possible murder.

Before he was arrested, the suspect told investigators that on his day off Monday he slept in the house with the three at night, and that they were still sleeping when he left for work around 6:45 a.m. Tuesday, according to the police.

Autopsy results showed that the two children were dead by 5 a.m. Tuesday, contradicting his claim.

The wife’s sister, who lives nearby, made an emergency call to the police around 9:20 a.m. Tuesday, saying Yukiko had committed suicide. Both the sister and the sergeant told investigators that Yukiko had worries about raising the children

The sister visited the victims’ home after receiving a call from the children’s elementary school saying they had not showed up to class.

Yukiko Nakata was found lying face up in the kitchen on the ground floor and her two children in their bedroom on the second floor. There were traces of burnt coal in the kitchen, indicating someone attempted to make it look like murder-suicide, investigative sources said.

The family’s house is in a residential area about 400 meters east of Nishitetsu Ogori Station.