The 43-year-old suspect had “turned himself in” on Sunday night, “admitting his involvement in incidents related to the triple murder in a house in Morata de Tajuña”, a police statement said.

Police had on Thursday found the three bodies, partially burnt and left in a pile inside their home in the village, which lies about 35 kilometres (20 miles) southeast of Madrid.

Neighbours raised the alarm after not seeing the two sisters and their disabled brother for some time, with police saying their deaths were being treated as murder over a suspected debt.

Police said Monday that the man, referred to only as D.H.F.C, was the “main suspect” in the case as he had “previously injured one of the female victims last year”.

Quoting local residents, Spanish media said the tragedy was likely linked to a fake online love affair, with the two sisters embarking on what they thought was a long-distance relationship with two alleged US servicemen.

They were led to believe one had died and that the other needed money so that he could send them a multi-million-dollar inheritance, causing the sisters to rack up huge debts.

Initially they began borrowing money from neighbours with the town’s mayor Francisco Villalain telling Spanish media they had rented out a room in their home to the Pakistani suspect for several months.

During that time, the suspect had reportedly lent them at least €50,000 ($55,000) which they had never repaid, prompting his violent attack on one of the sisters for which he was briefly jailed.

“They weren’t asking for €100 or €20, they were asking you for €5,000 or €6,000,” one neighbour had told TVE on Friday.

The sisters sent the money because the surviving soldier had promised seven million euros in inheritance money and refused to believe it was a scam, another neighbour told TVE.

Police did not comment on the reports.