State police said Matthew Slocum, 23, is the son of a woman who died in the fire Wednesday in White Creek, 35 miles northeast of Albany.
Slocum, an ex-convict, was allegedly holding his girlfriend, Loretta Colegrove, 25, against her will along with their son, Raymond, police said in Amber Alerts issued in New York and New England. The Amber Alert ended around 10 p.m. Wednesday.
Slocum was found in the general area of Banks St., Banks Ct. and Maguire Court in Gilsum, NH,and taken into custody.
Authorities say they have also located the 2003 Mustang he was seen driving.
The 4 a.m. fire killed Slocum's mother, Lisa Harrington. Also killed was her husband, Dan Harrington, a deputy highway superintendent, and his son Josh. Police said the fire might have been arson.
"They were good people. Dan was a real good employee," White Creek town Supervisor Bob Shay told the Glens Falls Post-Star. "We all feel horrible. It's just so sad."
He said Harrington and his wife served on the town recreation committee.
Slocum went to prison in 2006 for breaking into a house and stealing cash, a New York prison spokesman said. He was released but returned to custody twice for violations before his parole expired in December 2009.
Guns known to be in the home couldn't be accounted for, and police told the Post-Star they were asking neighbors whether they heard shots before the fire.
On Wednesday afternoon, emergency crews remained on the scene of the charred remains of the house, set back from a remote country road on heavily wooded property.
Police have not released any information about the cause of the fire.