Firefighters say it started just before 4:30 a.m. on South Humboldt Street near 2nd Avenue.
Neighbor Rolando Escobar described the flames: "Big, big. It was big. They started breaking down, putting water on it."
Firefighters went into rescue mode after initial knockdown of the flames at the front of the house, looking for a person inside a bedroom.
86-year-old woman killed in house fire on S. Humboldt St in San Mateo. 6 people displaced. Investigation into cause continues on scene. pic.twitter.com/rqUre35Xuv— Matt Keller (@MattKellerABC7) February 19, 2019
An 86-year-old woman was pulled out.
Firefighters did CPR, but could not resuscitate her. She died at the scene.
Six others, described as family members, have been displaced.
The Red Cross is helping them with food and housing.
Police and firefighters are investigating the cause. The coroner is also doing an investigation on the victim.