Updated

A New York man's mother, stepfather and stepbrother were sleeping when he killed them with a shotgun inside the family's rural upstate home six years ago, a prosecutor said Monday during the start of the defendant's retrial on triple homicide charges.

Eric Galarneau, a special prosecutor at the Washington County court trial, said during opening statements that Matthew Slocum's actions in July 2011 were "a nightmare of his making."

The charges against the 29-year-old Slocum include second-degree murder and arson. He's on trial for the slayings of his mother, Lisa Coon Harrington, 44, her husband, Dan Harrington, 41; and Harrington's son, Joshua O'Brien, 24.

Authorities said he used a shotgun to kill all three while they slept inside their home on a back-country road in the town of White Creek, on the Vermont border, then set fire to the house. Slocum and his girlfriend, Loretta Colegrove, then fled to New England, where Slocum was arrested in New Hampshire.

Galarneau said Colegrove witnessed Slocum shooting his stepbrother. Slocum's public defender, Michael Mercure, said during his opening statement that Colegrove actually carried out the killings because the Harringtons had planned to seek custody of the child she had with Slocum because of Colegrove's drug problems.

"You will hear and you will see that she is the real murderer," Mercure said.

Colegrove wasn't charged and plans to testify against Slocum.

A state appeals court tossed out Slocum's conviction in 2015 after ruling a statement he gave to investigators shouldn't have been shared with the jury that convicted him of murder.