CHICAGO — An escaped convict allegedly murdered an elderly woman as part of a bizarre plot aimed at assassinating US President Barack Obama after a cross-country killing spree, court records showed.

Maybelle Schein, 75, was discovered with her throat cut in the bedroom of her South Dakota home on Saturday afternoon.

Police used the OnStar tracking device in her Buick to find the car driving on a highway hundreds of miles away near Madison, Wisconsin.

James Vernon McVay, 41, told police of the plot after he was arrested in the woman's car after a brief chase and standoff.

His plan was to kill someone in Sioux Falls and steal their vehicle, then drive to Madison where he would "either break into another house to kill the occupants or would ambush and kill a Madison (WI) police officer to steal the offer's guns," charging papers filed Tuesday said.

He planned to keep killing people and stealing cars until he made it to Washington where he would "use a rifle to kill the president."

"The defendant stated 'I'm going to kill and kill until I get him,'" and told police "it didn't really matter who or when, but (he) was going to kill people whenever the opportunity arose," the charging papers said.

McVay said he chose Schein's house because the garage door was partially open. He slipped underneath and then made his way through the house where he Schein sleeping in bed and her purse on the kitchen counter.

"I didn't have to do it because I saw the purse. I could have just take the keys and cash and left but I didn't want to," he allegedly told police, adding that he killed her with a stolen knife because he needed to "get blood on (his) hands."

Schein's friends and family were struck by the senselessness of her death.

"We keep looking for answers, but you don't know why because no matter what the answers are, it doesn't change anything," close friend Marge Anderson told the Sioux Falls Argus Leader in a story published Wednesday.

"It's not gonna make sense. But it's human nature that you'll want to put the pieces together."

McVay has only one violent conviction -- an assault in Texas -- in his "extensive criminal history," the paper said.

McVay, who walked away from a halfway house on Thursday, has been charged with premeditated murder and burglary and could face the death penalty if convicted.