PALATKA, Fla. -- The 15-year-old girl whose disappearance triggered a nationwide Amber Alert was last seen Thursday in Texas riding in a pickup truck stolen from a handicapped man found slain in his Putnam County home. Investigators also say they found a call made from the dead man's home to the girl's cell phone.

Late Thursday, the Florida Department of Law Enforcement issued an Amber Alert for Morgan Amanda Leppert, who was reported missing from San Mateo, in southern Putnam County, on April 22. The Putnam County Sheriff's Office believes she is with her 22-year-old boyfriend, Toby Lowry, and may be in danger.

Thursday night, Putnam County named Lowry a person of interest in the death of 66-year-old James Thomas Stewart. Authorities found him dead in his Melrose home Thursday morning.

The sheriff's office told Channel 4's Dan Leveton that Stewart died of a "violent confrontation," but they did not want to be more specific on the cause of death because it remains part of their investigation.

Putnam County Sheriff Dean Kelly said that early in the investigation into Leppert's disappearance, officers tracked a call to the girl's phone from Stewart's home and deputies visited the house. Getting no answer at the door, they left a business card. When they returned Thursday and found Stewart dead, they immediately made the connection and became very concerned for the missing girl.

"We were on it, looking diligent for her, but just like any investigation, you develop new information, it turns things up -- it turns the priority of things up," Kellly said.

When deputies discovered Stewart's silver, four-door 2003 Toyota Tacoma pickup truck was missing, they entered it in a national crime information database. Putnam County investigators said they received reports the vehicle had been sighted three times on Interstate 10, with the latest report coming at 12:30 a.m. Thursday in Katy, Texas.

"We're not certain of (their location) at this point," PCSO Capt. Rick Lashley said. "She could be anywhere in the United States right now."

Putnam County has issued warrants for Lowry, charging him with grand theft of the vehicle and interference with custody of a child. Leppert remains the subject of a nationwide Amber Alert.

Leppert has blonde hair and blue eyes, is 5 feet 1 inch tall and weighs 114 pounds.

Lowry is described as having blond hair and green eyes. He is 6 feet 2 inches tall and 190 pounds.

Putnam County deputies found Steward dead in his Cherry Laurel Drive home after getting a request to check on his welfare.

Channel 4 learned that Lowry has a criminal history and was released last summer after serving a 20-month sentence for burglary and criminal mischief in Clay County.

On Lowry's MySpace Web page, last updated April 22 -- the day Leppert was reported missing -- he calls himself "Baddest m*****f***** in Palatka." Among the images on the page are pictures of guns of guns and Satanic references.

Lowry's page also contains posts from Leppert, calling herself "Blue-eyed Devil," professing her love for him. "Everything I wanted, I found it all in you," she wrote.

Channel 4's Dan Leveton was told that Leppert's mother found out Leppert was 22 years old and forced the girl to break off the relationship.

"Apparently there's been some visiting going on the mom didn't know about, then, all of a sudden, the girl goes missing," Kelly said.

Anyone with information about either Leppert or Lowry is asked to call the Putnam County Sheriff's Office at 386-329-0800 or the FDLE Missing Children Information Clearinghouse at 888-FL-MISSING. Anyone who sees either of them or the truck is asked to call 911.