Local television images showed large clouds of thick, black smoke billowing from the roof near a Macy's store. The mall -- one of the most popular in the region -- had been evacuated earlier in the day after a man barricaded himself inside a shop and started what appeared at first to be a small fire.
Authorities had thought the mall's sprinkler system had doused the blazed.
"We've got smoke coming from the building and we're positioned to get in there right now," city spokeswoman Vonette McCauley said.
There were no immediate reports of injuries.
Before the blaze erupted, police had detained a suspect and were checking a backpack they had found to see if it contained explosives.
They descended on the mall, 17 miles east of Sacramento, after a man claiming to have a gun entered the Electronics Boutique inside the Roseville Galleria Mall shortly before 10:30 a.m. Thursday.
He started a fire, then barricaded himself in a back room, but there were no reports of hostages or injuries, said Megan MacPherson, spokeswoman for the city of Roseville. The mall was evacuated.
She said the man is believed to have acted alone. Police did not know a motive, she said. They were not immediately releasing the suspect's name before he was taken to the Roseville city jail.
Roseville Fire Department spokesman Dennis Mathisen earlier said that fire sprinklers seemed to be controlling the flames, but firefighters had been unable to enter the mall because members of the police bomb squad were examining a backpack left behind by the suspect.
"I have no idea how this fire erupted again," Roseville Police Lt. Mike Doane said. "It is very much an active fire at this point."
The fire was reported flaring 50 feet or more through the roof, far from where the flames originally started in an Electronics Boutique.
Police and sheriff's deputies, including a county SWAT team, detained the man inside the mall. They were unable to immediately say if the man had a gun.