The boy was booked into juvenile detention on suspicion of aggravated murder. The girl was booked into detention on suspicion of tampering with evidence, said Detective Dennis McGowan.
The Salt Lake Tribune generally does not name juvenile offenders.
According to probable cause statements, witnesses told police that the Karrs and the juvenile got into a fight with Yazzie. Adam Karr pulled out a knife and repeatedly stabbed Yazzie, police wrote.
The Karrs then allegedly gathered bloody clothing and the knife and removed it from the home, which police said was at 31 E. Girard Ave. The Karrs fled the scene and concealed the evidence at another location, according to the probable cause statement.
Meanwhile, the juvenile dragged the bleeding victim out of the house to a nearby alley, where he "stomped on the victim's face," the statement reads.
The juvenile then returned to the home and concealed evidence by washing the bloody clothing, police wrote.
Resident Bert Aubrey has lived for 43 years just south of the home located at the intersection of Girard Avenue (560 North) and Desoto Street (40 East), where police believe the stabbing occurred.
Aubrey said the neighborhood, a block north of the state Capitol, was quiet and everyone had nicely groomed yards until about five months ago, when a college student began living in the house and started hosting parties with alcohol and large numbers of visitors.
Aubrey said he awoke at 3:30 a.m. Tuesday to flashing red and blue lights. He saw police cars and was initially pleased to see something being done about his raucous neighbor who often holds parties at the home.
"And then I saw the [crime scene] tape and I said, 'Oh, no,'" Aubrey said.
Sgt. Shawn Josephson said a 911 call came about 1:30 a.m. from "an inactive cell phone" that was disconnected except for 911 service. The caller "indicated there was a man down and bleeding in the area," Josephson said.
The victim was found in an alley a half-block southwest of the home bleeding from multiple stab wounds. The victim "had suffered stab wounds and was conscious at the time," Josephson said, adding that he was transported to a local hospital where he died soon after.
Police followed a blood trail from the victim's location to the side door of the home.
Officers also found blood inside the home along with a 17-year-old boy. Detectives questioned him and later handcuffed two other males at the scene.
"We have picked up a couple of people for questioning," Josephson said. "We also are seeking some other people we believe may have information important to the case. There are currently two females we're looking for ... believed to be witnesses [to the stabbing]."
One of the women, known only as "Marissa," is believed to be about 20 years old, 5-foot-3, 110 pounds with shoulder-length blonde hair; the second female, known only as "Juliette," was about the same age, 5-foot-6, 110 pounds, with brown hair falling to her back. They were last seen driving a blue car.
Police had no documented reports of prior problems at the home. Josephson said a preliminary check of records did not turn up any police reports pertaining to the address, but he said that doesn't mean police haven't ever responded to the home.
But neighbors said the home its front yard covered in tall yellowed grass and littered with beer cans has been a nuisance.
"It has been a problem house for a while," said neighbor Vince Kerzman.
Kerzman said lots of his neighbors have called police about the home in recent weeks complaining about noise and late parties.
"All of us have called the cops [on the activity at the house]," Kerzman said, adding that residents knew something like the fatal stabbing eventually would occur.
Adam Karr was wanted on a $5,000 warrant for failure to appear in 2nd District Court in Farmington after pleading guilty in 2011 to sexual abuse of a minor, a third-degree felony.
According to court documents, police found Adam Karr shirtless with his pants down in the backseat of a car with a 15-year-old girl. The car was parked at West Bountiful City park, where he had smoked marijuana and spice with the girl. He then removed his shirt, asked her to rub his back and requested multiple times to give him oral sex, according to court documents. Drug charges were dropped as part of the plea agreement.