Investigators spent Tuesday exploring the links between the suspect, Mark Geisenheyner, and Paul and Monica Shay, a New York city couple.
They also will pursue Geisenheyner's claims that a 2006 fire at the Shay home was an arson tied to an alleged insurance scheme.
Geisenheyner, 51, stormed into the Shays' rural weekend retreat outside Philadelphia late Saturday and shot five people in the head, authorities said. Shays' adult nephew and a visiting 2-year-old boy died, while the Shays and the boy's mother were critically wounded.
Geisenheyner was killed by police after a standoff Monday.
He had vowed revenge after Paul Shay allegedly cheated him out of his full share of the proceeds from an insurance scam, his acquaintances told police.
A career criminal, Geisenheyner had been arrested in September 2006 and charged with receiving 16 wildlife prints that Paul Shay had reported stolen from the home. That charge was later dropped when witnesses failed to appear, North Coventry Township Police Chief Robert Schurr said Tuesday, citing department records.
Geisenheyner by then had been sent back to prison on a parole violation, Schurr said.
A girlfriend found the prints at the apartment she shared with Geisenheyner, the police report said. Detectives in Douglass Township, where the Shays lived, were the lead investigators.
Douglass Township Police Chief Barry Templin Jr. called the art theft investigation an open case on Tuesday and declined to provide any details.
Paul Shay remained hospitalized Tuesday. He has extensive injuries but talked to police about the shooting and was expected to survive.
The large fire caused significant damage to the couple's home, which they largely rebuilt but continued to work on, neighbor Kath Landis said.
The 64-year-old Shay owns a plumbing company and lives in Manhattan's East Village with his 58-year-old wife, who works as the director of the Pratt Institute's arts and cultural management program.
His slain nephew, 43-year-old Joseph Shay, had lived with the couple in lower Manhattan since his release from prison about a year ago. Joseph Shay's girlfriend, 37-year-old Kathryn Erdmann of Fall River, Mass., and her toddler, Gregory, were joining them at the vacation home for the holiday weekend.
Kathryn Erdmann called 911 to report the shooting, investigators said. Her son was in bed when he was shot in the head.
"It's a haunting call," Assistant Montgomery County District Attorney Kevin Steele said. "She'd been shot, she's covered in blood, she's trying to get help for her son."
Landis and her family were home Saturday night, with the windows open and the TV off, but didn't hear anything except some nearby fireworks until police and ambulances arrived.
"It's a horrific tragedy that somebody can just flip into somebody's home and take so many lives. It's just sickening," Landis said.
Investigators plan to review the cause of the fire and any insurance claims filed, Montgomery County District Attorney Risa Ferman said Tuesday. About a month after the fire, Paul Shay reported to police that artwork and other valuables were missing from the home.
"We'll track it back to find out exactly what happened," she said.
John Penley, a freelance photographer in New York, finds any fraud allegation incompatible with the man he knew.
"I do not ... think Paul would be dumb enough to jeopardize his whole business for, in the scope of things, not a lot of money in an insurance scam. I don't believe it," Penley said.
About 75 people gathered Tuesday night for a vigil in front of the Shays' apartment in Manhattan, placing bouquets and candles on the small stoop and sharing stories about the couple. Sally Block, one of Monica Shay's colleagues from the Pratt Institute, said she had received more than 200 e-mails from students, alumni and colleagues expressing shock and grief.
"I'm just wishing for health, safety and peace for her and her family," Block said.
Associated Press Writer Karen Zraick in New York contributed to this report.