Mass murderer Adam Lanza spent two and half days home alone with his mom’s arsenal of weapons, giving him plenty of time to hatch one of America’s worst school shootings, according to broadcast report today.

Before Lanza slaughtered 20 kids and six educators at Sandy Hook Elementary School in Newtown, Conn., on Friday, he killed mom — an avid gun collector — Nancy Lanza in their home that morning.

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Nancy Lanza had just arrived home early Thursday evening, following a two-night getaway to Bretton Woods, NH, according to Headline News.

She checked into the Omni Mount Washington Resort at 12:10 p.m. on Tuesday, Dec. 11, and checked out at 12:27 p.m. on Thursday, HLN said.

It’s 290 miles — nearly a five-hour drive — between Bretton Woods, NH, and Newtown, Conn.

Lanza had visited Bretton Woods before and had always driven herself there, according to HLN.

She had chatted with friends during her two nights in New Hampshire and appeared to be in good spirits, the cable news network reported.

Nancy Lanza often took mini-vacations by herself, friends said. She trusted leaving Adam alone, but didn’t want him cooking, so she always prepared all his meals before her trips.

Yesterday, it was reported that Adam Lanza studied photos of guns and obliterated virtual victims in violent video games for hours on end, alone in his windowless basement den.

The unhinged Sandy Hook Elementary School gunman was enthralled by blood-splattering, shoot-’em-up electronic games, which he played in the basement of his mother’s spacious $1.6 million home in Newtown, Conn., according to a person familiar with the layout.

Lanza, 20, especially liked “Call of Duty” — a wartime role-playing game where participants use high-powered assault rifles, machine guns and other weapons to slaughter scores of people, according to a published report.

And the basement walls were covered with military- and weaponry-themed posters, said one of the home’s few visitors.

“It was a beautiful house but he lived in the basement. I always thought that was strange,” said plumber Peter Wlasuk, who did work on the house and saw Lanza’s subterranean lair, The Sun newspaper of Britain reported.

“They had one poster of every piece of military equipment the US ever made,” Wlasuk said.

“The kids could tell you about guns they had never seen from the 40s, 50s and 60s,” the plumber said of Lanza and his older brother, Ryan, who had also lived in the basement before moving away for college.

Once Ryan moved out, “Adam then moved down there. The boys were fans of the military. They had posters all over the wall in the basement,” the plumber said.

“The kids who play these games know all about them. I’m not blaming the games for what happened. But they see a picture of a historical gun and say ‘I’ve used that on Call Of Duty.’”