MELROSE, Fla. -- North Florida authorities charged a 15-year-old girl, who was the subject of an Amber Alert, and her boyfriend with fatally beating, stabbing and suffocating an older man.

Putnam County Sheriff Dean Kelly said Tuesday at a news conference that a first-degree murder warrant had been issued for Morgan Amanda Leppert, who is in custody with her boyfriend in El Paso, Texas, charging her with the slaying of 66-year-old James Stewart.

State Attorney John Tanner said it will be up to a grand jury whether to charge Leppert as a juvenile or adult.

Her boyfriend, Toby Lowry, 22, will be extradited on charges of grand theft auto for stealing Stewart's pickup and interference of custody. Lowry will be charged with first-degree murder when he returns to Florida, authorities said.

Authorities did not know if Leppert or Lowry had lawyers.

The couple was picked up in El Paso Saturday evening when a motorist spotted them after seeing the Amber Alert and called police.

The 2003 Toyota Tacoma pickup that authorities said they stole from Stewart was found nearby. He was found dead Thursday in his home in Melrose, about 20 miles east of Gainesville. Police also believe they took money from Stewart.

Stewart was beaten with two metal rods, stabbed several times and suffocated with a plastic bag over his head.

Putnam County investigators had been searching for Leppert since she was reported missing on April 22. The case was elevated to an Amber Alert when deputies made a connection between her disappearance and Lowry.

Officials said Lowry has a lengthy criminal history and was released from prison last summer after serving a 20-month sentence for burglary and criminal mischief.

Putnam County investigators interviewed Leppert and Lowry in Texas on Sunday.

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