A British man held over the deaths of his American wife and baby daughter at their US home may have been planning to also kill himself, US authorities say.
The Entwistles had moved to the US after their daughter was born
Neil Entwistle, 27, was remanded in custody by magistrates at an extradition hearing in London following his arrest in the city.
He faces charges of murder and illegal possession of a firearm in the US.
The bodies of Rachel Entwistle, 27, and Lillian, nine months, were found in Hopkinton, Massachusetts, last month.
In a press conference in the US on Thursday, Middlesex District Attorney Martha Coakley said Mrs Entwistle had spoken to members of her family on the night of 19 January.
Ms Coakley said the following morning Mr Entwistle had shot his wife and daughter, as they lay in bed, with a gun he had obtained from his father-in-law Joseph Materazzo.
"We believe possibly that this was intended to be a murder-suicide, but we cannot confirm that. The murder was effected, the suicide was not."
Mr Entwistle left the US after the deaths and had since been staying at his parents' home in Worksop, Nottinghamshire.
The family of Mrs Entwistle later said in a statement they were "deeply saddened" by his arrest.
"We are heartbroken at the loss - to understand how this happened."
Ms Coakley said forensic evidence linked the gun to Mr Entwistle and his wife.
She said Mr Entwistle returned the gun to his father-in-law's home, before making preparations to leave the country.
He then bought a one-way ticket back to the UK and left at 0815 local time from Logan airport, in Boston, on 22 January, the prosecutor said.
"Authorities went to the UK to pursue that investigation. We explored motive, with witnesses, with information from searches, internet capabilities."
She said this information, combined with forensic evidence linking Mr Materazzo's handgun to the couple, gave "probable cause" to seek Mr Entwistle's arrest.
Ms Coakley said Mr Entwistle had had debts in England when he moved with his family to the US and that his new internet businesses were failing.
She added that he also had new financial obligations, including the lease on his five-bedroom home.
The bodies of Mrs Entwistle and Lillian were discovered under a pile of blankets in a bedroom at the home on 22 January.
Mr Entwistle was arrested by Metropolitan Police officers at Royal Oak Tube station, west London, on Thursday.