A teenager accused of killing a man and wounding another in a series of highway shootings had argued with relatives during a hunting trip and drove off in anger shortly before the attacks, a prosecutor said Wednesday.

Zachariah Blanton, 17, pleaded not guilty Wednesday to charges of murder, attempted murder and three counts of criminal recklessness. He stared at the sidewalk as officers led him into the Jackson County Courthouse where Circuit Judge William Vance set a tentative Dec. 13 trial date.

Jackson County Prosecutor Stephen Pierson said after the hearing that he would decide by Blanton's next court date of Oct. 13 whether to pursue the death penalty.

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Blanton's grandmother, his legal guardian, was also charged with obstruction of justice related to the rifle used in the shootings, authorities said.

Police believe that after the hunting trip argument, Blanton drove to a nearby overpass outside Seymour early Sunday, aimed his rifle over the trunk of his vehicle and fired at trucks on Interstate 65 about 60 miles south of Indianapolis.

One bullet killed a passenger in a pickup and another wounded a man. Two hours later, a tractor-trailer and unoccupied sport utility vehicle were hit by bullets about 100 miles to the north, close to the teenager's hometown of Gaston.

"He said he in fact did do the shootings, however, only with the intention of relieving pressure," John Kelly, an Indiana State Police crime scene technician, testified at a hearing Tuesday.

The teen apparently had been arguing with relatives over gutting a deer during a hunting trip about 20 miles south of Seymour, Pierson said.

When investigators questioned him Tuesday after receiving a tip that he might have been involved, his demeanor was "cooperative, it was remorseful," said Sheriff George Sheridan Jr. of Delaware County, where the second set of shootings occurred.

The teen lived with his grandparents in Gaston, where detectives found the rifle they believe was used in the shootings, authorities said. Blanton's grandmother, Patricia Blanton, was charged Tuesday with obstruction of justice.

Sheridan said the 58-year-old woman's charge related to the rifle but would not say if she hid it or knew where it was.

Zachariah Blanton came to the attention of investigators on Monday after an acquaintance told a Delaware County reserve deputy that the teen might be involved in the shootings, State Police Superintendent Paul Whitesell said. Pierson said police had recovered a rifle they believed was used in the shooting death of Jerry L. Ross, 40, of New Albany.

A second attempted murder charge could be added because a bullet might have grazed the head of a passenger in Ross' vehicle, the prosecutor said.

Blanton had previous brushes with the law for what Sheridan would describe only as "crimes of sex and of theft." On his Internet page on MySpace.com, he listed his interests as "Huntin, Fishin, Muddin and goin out with friends."

There was no immediate response to a call seeking comment from Blanton's grandparents, who police said were his legal guardians.

Blanton's great-aunt Denise Blanton told The Star Press of Muncie that she was shocked by the allegations.

"I can't imagine that he would be involved," she said.

Kenneth Shipley, 59, a family friend who lives in Gaston, said he was also surprised.

"This isn't the Zach I knew," he said. "I took this real hard. Zach just needed some attention, someone to talk to."