PALATKA, Fla. -- Three days after a 15-year-old girl at the center of a nationwide Amber Alert and her 22-year-old boyfriend were picked up in Texas, Putnam County Sheriff Dean Kelly announced that both are charged with murder in the death of a 66-year-old Melrose man last week.

Morgan Leppert and Toby Lowry were picked up in El Paso just after 4 p.m. Saturday when a motorist recognized the couple from seeing a national news report and called 911.

Lowry and Leppert remain in El Paso, Texas, where Lowry was being held on charges of car theft, interference with custody of a child and panhandling. Leppert does not face charges in Texas, but had been held in protective custody by juvenile authorities since Saturday night.

Kelly announced that Lowry and Leppert were both being charged with first-degree murder and would be returned to Florida as soon as possible.

Kelly said that Stewart was beaten with two metal rods, stabbed several times and suffocated with a plastic bag over his head. The details of the killing were withheld from the public until after investigators had a chance to interview Lowry and Leppert.

Investigators said there was no prior relationship between the couple and Stewart, who was disabled and hard of hearing.

"They were looking, basically, something to get into, for a way to get out of the country," Kelly said. "They stayed in a wooded area behind the victim's home. They just cased the area looking for an easy mark."

Investigators said Leppert knocked on Stewart's door, said her car had broken down and asked to use the phone. That call was made to her own cell phone -- the connection between her disappearance and the homicide that led Putnam County to issue the Amber Alert.

"Of course, there are always suspicions, but until we had more facts, we had to error on the side of caution and treat her as an endangered missing child," Kelly said.

Kelly praised the national news media for getting out the Amber Alert information, which resulted in the capture of two people now charged with murder.

Leppert, Lowry and a third person who officers believe was a hitchhiker from Florida were panhandling just off Interstate 10 when a man who saw the Amber Alert coverage that morning on the Fox News Channel.

The stolen 2003 Toyota Tacoma pickup they allegedly stole after killing Stewart was found nearby.

Putnam County investigators had searched for the San Mateo teenager since her mother reported her missing on April 22, but the case was elevated to an Amber Alert when deputies made the connection to the homicide.

Lowry has a lengthy criminal history, with arrests in both Putnam and Clay counties. He was released last summer after serving a 20-month sentence for burglary and criminal mischief.

Deputies received three reports of the stolen vehicle headed west on I-10, the most recent was early Thursday morning near Houston.

By noon Sunday, three Putnam County investigators were in Texas interviewing Lowry, Leppert and the hitchhiker, Robert Brucker, 43. Investigators said the couple picked up Brucker in Valdosta and had no connection to the slayings.

Kelly said that, ironically, the fact that Leppert was 15 is probably the reason she and Lowry were caught quickly. If she had been 18 -- too old to qualify for the Amber Alert that generated the national news coverage -- they have not idea if or when authorities would have caught up with Leppert and Lowry.