Fire officials determined after their daylong investigation that the blaze was started accidentally by a burning candle at the center of the memorial structure.
Central Fire Authority Fire Chief Doug Williams, who oversaw the investigation, said no evidence of accelerants, such as gasoline, was found at the site.
Williams could only speculate that the candle was somehow knocked over, perhaps by a gust of wind. "It's an open flame is what it amounts to," he said.
Seven arson investigators from four agencies -#8212; Cal Fire, Sonoma County Fire, Central Fire and Rincon Valley Fire -#8212; examined evidence at the scene Thursday. The number was unusual for a fire of a such a small size and reflected the sensitive nature of the memorial site.
"There's a lot of meaning attached to it. We wanted to be sensitive to that," Williams said.
Lopez was shot Oct. 22 by Deputy Erick Gelhaus as the boy walked through his southwest Santa Rosa neighborhood carrying an airsoft BB gun designed to resemble an assault rifle. Gelhaus told investigators he thought the boy was carrying an AK-47.
Within days, those both mourning Lopez and protesting his death built a 19-foot-long wooden shrine in a vacant lot at the site of the shooting. The memorial, which was decorated with candles and flowers, resembled a type of grave marker common in Mexico.
Caretakers of the site said electricity had been cut off for about a month from the strings of lights that hung around the tent and altar, only because a neighbor could not keep pace with the electricity bills.
Further, they said, candles that once burned at all hours had been limited to a few prayer candles positioned near the sidewalk -#8212; about 20 feet from where the altar burned.
Regular visitors to the site said several photos of Lopez were missing after the blaze, further fanning their suspicions that it was arson.
Williams said that firefighters did not confiscate any photos from the scene.
Jonah Colon, 13, who attended Cook Middle School with Lopez, arrived at the site before 9 a.m. Thursday morning.
"There was still steam coming off it," he said. "It was a bad sight. My friend was shot down right here, and it was just burned."
More than 20 people were at the site Thursday afternoon, using shovels, ladders and other tools to repair the structure covering the majority of the memorial.
The 400-square-foot tent that has stood over the site for weeks was marred by a huge hole with singed edges. The white, particle-board altar, once covered with photos and messages, was charred black and torn apart.
Bags of melted strings of lights, burned toys and broken vases were piled around the area that once spelled Lopez's name in illuminated prayer candles.
"It's not going to affect the community," said Jess Perez, who has been working with teens that have come together since the shooting, calling themselves Andy's Youth.
"We are just going to become stronger," she said.
Negotiations to turn the 1-acre lot where Lopez was killed into a park are ongoing. The property is owned by real estate agent David Poulsen.
"Supervisor (Efren) Carrillo is working on some negotiations with the owner and Regional Parks and the Open Space District, who are also part of the discussion," said Sonoma County Supervisor Shirlee Zane, who was at the site Thursday afternoon.