Jack Dorsey saddened by Japan's 'Twitter killer'

Takahiro Shiraishi was arrested after police found several people's body parts in his home

Twitter's chief executive has said a case in which an alleged serial killer is said to have lured victims by searching the social network for suicidal thoughts is "extremely sad".

But Jack Dorsey added that it was not realistic to expect the service to auto-delete the kind of tweets said to have been involved.

A 27-year-old man was arrested in Zama, Japan, in October after nine people's body parts were found in his flat.

Newspapers call him the Twitter killer.

Local reports claim Takahiro Shiraishi contacted his victims - the youngest of whom was 15 - via the social network by telling them he could help them die and in some cases claiming he would kill himself alongside them.

His Twitter profile contained the words: "I want to help people who are really in pain. Please DM [direct message] me anytime."

'Positive and healthy'

He is also alleged to have killed the boyfriend of one of the women, who had apparently come looking for her.

According to various reports, Shiraishi has confessed his involvement to the police, but to date he formally faces only charges of abandoning a body.

Four days after his arrest, Twitter amended its rules to state members should not "promote or encourage suicide or self-harm".

But with the case in the spotlight, the Japanese government has indicated it may introduce new regulations to tackle "problematic" websites on which suicide is discussed.

Mr Dorsey gave his first interview about the affair to Japanese broadcast NHK.

"We need to take on a responsibility to make sure our tool is being used in positive and healthy ways," he said.

But he added that simply deleting suicidal comments would not prevent people killing themselves and that helping connect the right kinds of people could help.