A fast-moving grass fire burning east of Livermore has charred at least 2,500 acres, Cal Fire said late Wednesday.
The blaze was 5 percent contained.
Reports of the fire came in at 2:45 p.m. off Tesla Road near Corral Hollow Road between Tracy and Livermore, Cal Fire officials said.
Shortly after 4 p.m. the fire covered about 100 acres, and shortly before 5 p.m. it had spread to about 200 acres, according to Cal Fire.
At 5:20 p.m. the fire had been reported at 500 acres but by 6:20 p.m., it had already doubled in size to 1,000 acres.
As of 7:37 p.m. the fire had grown to 1,500 acres, Cal Fire said, but by 8:25 p.m. the fire had reached 2,500 acres.
Cal Fire spokeswoman Pam Temmermand said the fire has been growing steadily since it was first reported in the afternoon.
"If the wind dies down, maybe things will slow down a bit," Temmermand said. "But I know they're getting gusts of 12 to 13 (mph), so we'll see."
The Carnegie Motocross Park at 18600 Corral Hollow Road was evacuated, according to Cal Fire.
No injuries have been reported, but Cal Fire officials said one structure has been lost so far. Details about the kind of structure involved were not immediately available.
Fire crews plan to remain on scene through the night and into Thursday, according to Temmermand.