In a prison cell interview, Sikander Zia told Fairfax he and a gang of hired assassins killed the family's seven-year-old son last, binding him with rope and strangling him to death with wire after all of his family members had been murdered.

“I could not let him live, he knew me and we had played together, and he could identify me,” Zia said in English.

Zia has been arrested by Islamabad police and will be charged with the murders of five members of his own family: Amir Khan, Mr Khan's wife Nazia Amir, and their three children, Romana, 17, Adam, 14 and Haider, 7.

Twenty-seven-year-old Zia, an MBA graduate, was a nephew to Nazia Amir, and lived near to the family's home in Bahria Town, in southern Islamabad, where they were killed on Sunday, October 13.

Zia confessed to police, telling them he hired six assassins from his home province of Khyber Pakhtunkhwa, promising them 2.5 million Pakistani rupees to kill the entire Khan family.

His family stood to inherit much of Amir Khan's wealth, reportedly worth tens of millions and mainly held in property, if the Khans all died.

The five Khans were all dual Australian-Pakistan citizens. The two eldest children were born in Australia and the family still owned houses, and had bank accounts, in Australia.

Zia and his accomplices also stabbed to death the Khan's family servant, Asghar, who, police believe, interrupted the crime in progress last Sunday night. Zia has told police Asghar was paid 500,000 Pakistani rupees to assist them in the crime.

Sitting barefoot and wearing a blue salwar kameez, bound in handcuffs and blindfolded with a red sash,  Zia spoke to Fairfax in an Islamabad police cell.

He said he murdered the wealthy family for their money because his own petroleum business had foundered.

“It was my plan I came up with 15 days ago. I was facing financial crisis and this was the easiest solution to get money.

“I found the six men in Khyber Pakhtunkhwa and made an agreement with them that we would kill these people. I promised them that after the crime I will pay them 2.5 million rupees.”

He said his confession to police was not made under any duress: “I have not been put under any pressure.”

And he said he bore no animosity towards Khan or his family.

“There was no issue between us, between me and Mr Amir. He was a gentleman.”

Zia said he now expected, even wanted, the death penalty, which has only recently been reintroduced in Pakistan after a moratorium, but is still only applied sparingly.

“I cannot face anyone now, not my family, not my friends, not anyone. I can't be forgiven. If I go before a court I will ask the judge for the death penalty. That's what I want. Six people have died because of me, I must face what I did.”

Police Superintendent Mustanzar Feroz told Fairfax the Khan family allowed Zia into the house on Sunday night because he was a trusted relative and a regular visitor. Nazia Amir served him juice.

Zia then let in his accomplices, who attacked the family. The Khans were bound with rope and had their mouths taped shut before being strangled to death, one by one.

“It was a heinous and gruesome crime, the most inhuman act imaginable.

“All the family were killed in front of each other.”

Zia had been under police suspicion since the hours after the killing, but he was formally arrested on Sunday, a week after the murders. Three of his hired killers have been arrested, while three remain on the run.

Zia's mobile phone records show he visited the two sites in Islamabad where the family's bodies were dumped, and he was evasive and contradictory in his initial interviews with police, Superintendent Feroz said.

He said Zia confessed his motive was money. Cash, jewellery and mobile phones were taken from the crime scene.

“Amir Khan was a wealthy man, a billionaire by Pakistani standards. He owned 13 houses in Bahria Town, and he lived a luxurious lifestyle. Sikander's family was not well off, and they stood to inherit a large amount of Mr Khan's wealth if his family were all killed.”

Khan had been threatened before his murder and had bought a Kalashnikov. Friends in Melbourne had warned him not to return to Pakistan, saying he would be killed.

With Khudayar Khan

@SouthAsiaCorro