The defendant who will turn 18 on Friday is accused of shooting and killing 58-year-old Geraldine Bommarito on March 18 or 19.
Canfield is being held at the Washington County jail in lieu of $500,000 cash bail.
A 2010 Utah statute mandates that 16- and 17-year-olds charged with murder are under the jurisdiction of adult court.
Sheriff's deputies found Bommarito's body while investigating an unrelated burglary at a Toquerville home, where two firearms and collectable coins were taken on that morning.
While investigating the burglary, deputies identified the teen who is Bommarito's neighbor as a possible suspect.
On March 19, the teen was seen at a convenience store in Washington City, where he made a purchase using coins consistent with those taken in the burglary, according to the probable-cause statement.
Witnesses provided a vehicle description and license plate number for the vehicle driven by the suspect, which investigators traced back to Bommarito, the registered owner. When investigators went to the residence listed on the vehicle registration, they discovered the deceased woman, who had been shot at least once.
On the afternoon of March 19, detectives located Canfield in Springdale and took him into custody.
According to a probable cause statement, the teen was "belligerent and combative" with arresting officers. He was not booked into jail until three days later because he was receiving treatment at a medical facility, according to court records.
Washington County Sheriff's Chief Deputy Jake Schultz said Tuesday that Canfield has attempted to injure himself twice since he was arrested. Canfield apparently cut his neck before he was booked into the jail, and injured himself again just minutes after he entered the jail. That injury required a five-hour hospital visit and the use of a neck brace, according to Schultz.
The teenager is currently being kept in an isolation cell, Schultz said, and is checked by deputies every 15 minutes.
Canfield was in possession of two firearms that were consistent with evidence at the scene of the homicide, according to the probable cause statement.
Deputies also found a burned car in a remote area of Springdale which they believe may be Bommarito's. The vehicle was thoroughly burned and the license plate was no longer legible, according to police.
Neighbors have described Bommarito as a nice woman who ran a home cosmetics business and volunteered at nearby Zion National Park. She moved to Toquerville about five years ago and came from an Italian-American family in Las Vegas.