He has been charged with two counts of murder, one count of illegal possession of a firearm and one count of illegal possession of ammunition, said Melissa Sherman of the Middlesex district attorney's office.
Rachel Entwistle, 27, and 9-month-old Lillian, were found dead Jan. 22 in their home in the Boston suburb of Hopkinton.
Neil Entwistle, who authorities say used his father-in-law's shotgun to shoot his wife and daughter, may have intended to kill himself as well but did not follow through, London District Attorney Martha Coakley told the Associated Press.
Neil Entwistle, 27, was taken into custody in the UK by detectives and will face an extradition hearing. Since his family was killed he had been staying at his parents' home in Nottinghamshire.
Rui Vieira / AP
The couple originally met at the University of York in 1999 and lived in England until last year, when the American-born Rachel, who taught school in Worcestershire, said she wanted to live closer to her parents in Massachusetts. Their daughter was born last April.
On Feb. 1, Rachel and Lillian Entwistle were laid to rest together in a wooden casket at a service attended by about 500 mourners – but not Neil Entwistle. The memorial service was at the same Roman Catholic church where the baby girl had been baptized not two months earlier.