Neil Entwistle is on a plane back to Massachusetts to face charges of murdering his wife and daughter in Hopkinton.
Scotland Yard, which has had custody of Entwistle, a British citizen, since he was arrested in London last week, said he was transferred to US marshals at Gatwick Airport outside London at about 2:25 p.m. local time (9:25 a.m. EDT)
Once he arrives in Massachusetts later this afternoon, Entwistle, 27, is expected to be booked at the Hopkinton police station on charges of shooting Rachel, 27, and 9-month-old Lillian Entwistle to death on Jan. 20. He is scheduled to be arraigned in Framingham District Court at 2 p.m. tomorrow, the Middlesex District Attorney's office said.
The state's public defender agency has appointed Boston defense lawyer Elliot M. Weinstein to represent Entwistle, after his parents in England told the agency they could not afford to hire one on their own.
"I was asked to represent a person who has been accused of a serious crime and it's part of what I do -- represent people accused of serious crimes -- and that's why I'm representing Neil Entwistle," Weinstein said.
He said he has yet to speak with, or even meet, his newest client. "My hope is to meet with him and speak with him as soon as possible after he lands," Weinstein said.
Assistant District Attorney Michael Fabbri is set to be the prosecutor handling the case. Framingham District Judge Robert V. Greco is to preside over the arraignment. He has been criticized by some lawyers for releasing more than 150 pages of police affidavits that detail much of prosecutors' evidence that alleges he killed his family.
The day after the killings, investigators say, Entwistle flew to his native England, leaving the bodies together in a bed under a pile of blankets.
Rachel Entwistle's relatives called police after the couple organized a dinner party but did not answer their door when guests arrived, investigators said. Officers discovered the bodies on Jan. 22, and found Entwistle's white BMW sport utility vehicle parked at Logan International Airport.
Authorities outlined their case in search warrant affidavits, describing Entwistle as a secretive man who was sinking deep into debt. The documents say that in the days before the killings, Entwistle trolled the Internet looking for sexual partners and information on ways to kill people and commit suicide.
On Feb. 9 British authorities arrested Entwistle at a north London subway station on charges he killed his wife and daughter. He waived extradition and agreed to return to the United States.
Middlesex District Attorney Martha Coakley said authorities believe Entwistle used his father-in-law's .22-caliber handgun in the shootings, and may have planned to the turn weapon on himself because of his mounting debts.
Instead of committing suicide, Entwistle drove the gun back to his father-in-law's house, then fled to his native England, authorities allege.
Hopkinton Police Chief Tom Irvin has said he expects Entwistle to be booked at the Hopkinton police station and held there until his arraignment in Framingham District Court.
Entwistle met Rachel Souza, a Holy Cross student from Kingston, in 1999 at Britain's University of York, where she was spending the year abroad.
The couple married in 2003 and lived in England. In April 2005, Lillian was born. The couple moved to Carver in southeastern Massachusetts last summer and lived with Rachel's relatives.
In January 2006, the family moved into Colonial-style home they rented in Hopkinton, a rural town about 35 miles west of Boston.
Ten days later, mother and daughter were found dead.
Material from the Associated Press was used in this report.