A man was arrested from Sukkur's Pano Aqil tehsil for allegedly killing 11 members of his family, including six women and five children, police said on Wednesday.
Police said that the victims included the suspect's wife, four daughters, three sons, daughter-in-law and a grandson and granddaughter.
Speaking to the media, DIG Sukkur Fida Hussain Mastoi said that initial investigations suggested the victims' throats were slit with a knife as they slept during the night.
"The accused is none other than their father and the head of the household," he said. "We have been told that he was mentally unstable and was previously under treatment at a hospital."
He added that the suspect had also committed a murder prior to this. "We are in the process of collecting information. Who did he murder, when did he do it and how long did he spend in jail, we are collecting this [information].
"Obviously when a person is not mentally stable, what we have seen, is that they do such things," he said.
The official added that three of the suspect's sons had also been arrested and police had seized the murder weapon. "Further investigation is under way."
When asked whether the victims had been drugged before having their throats slit, the officer replied that this could only be ascertained once the chemical reports were received.
"What we can tell so far is that the victims didn't put up any resistance. But drugs may also be an element as they didn't put up any resistance and due to the sheer number of deaths," he said, reiterating that this could only be determined through the chemical reports.
Meanwhile, Sindh Chief Minister Murad Ali Shah took notice of the killings and sought a report from the IG.