"The origin of the fire was determined to be in the center of the memorial," Central Fire Authority of Sonoma County Chief Doug Williams said in a news release late Thursday night.
"Based on the findings of the investigative team, the cause was determined to be accidental, related to a candle located in the vicinity of the origin," Williams said.
Investigators conducted interviews and reviewed photos to reconstruct the scene and no accelerants were detected, Williams said.
The fire at the field at Moorland and West Robles avenues, where Andy was fatally shot by sheriff's Deputy Erick Gelhaus on Oct. 22, was reported by an anonymous caller at 12:08 a.m. Thursday.
Firefighters from the Todd Road fire station quickly extinguished the blaze and fire investigators remained at the site Thursday morning and afternoon.
The fire burned the 10-foot-long altar made of particle board, as well as a wood cross and a portion of the top of an unenclosed tent that housed the altar and cross.
Charred items at the scene included a toy car and truck, a religious statue and flowers.
Nicole Guerra, a community spokeswoman and one of the organizers of the marches and rallies that followed Andy's death, said community members would begin rebuilding the altar on Saturday.
The altar was erected in early November, Guerra said.
Andy was carrying a BB gun when he was shot seven times by Gelhaus, 48, a 24-year sheriff's office employee who was also a firearms training instructor.
According to Santa Rosa police, Gelhaus believed the BB rifle was actually an AK-47 assault rifle. Gelhaus said he told Andy to drop the rifle and shot him when the gun rose upward as Andy turned toward him, police said.
Gelhaus fired eight shots from behind the open door of his patrol car. A deputy who was with him, and who has not been identified, did not fire his weapon.
Gelhaus was on paid administrative leave until early December, when he returned to work in an office in the sheriff's office.
Santa Rosa police are investigating the shooting and the results of their investigation will be reviewed by the Sonoma County District Attorney's Office.
Andy's family has filed a civil rights violation lawsuit against the county and the sheriff's office. The suit alleges the fatal shooting was without cause or provocation and "a senseless and unwarranted act of police abuse."
Protestors have held marches from Santa Rosa City Hall to rally at both the sheriff's office and the county courthouse and jail and have also rallied and protested at meetings of the Santa Rosa City Council and Sonoma County Board of Supervisors.
Another protest is scheduled for Tuesday afternoon's supervisors meeting.
The supervisors are exploring whether the field where Andy was killed can be purchased from a private developer and become the site of a park in his memory.
The board also has created a Community and Law Enforcement Task Force that will make recommendations about a process for independent citizens' review of police conduct and officer-involved shootings.
The final appointments to the 21-member task force are expected to be made Tuesday.