Sky-darkening wildfires claimed at least six lives and forced tens of thousands of people from their homes blazed throughout California on Friday as firefighting resources strained under the vastness of the infernos authorities were trying to control.

Three major complexes encompassing dozens of fires chewed through a combined 780 square miles (2,020 square kilometers) of forests, canyons and rural areas flanking San Francisco on three sides.

The fire outside Vacaville, a city of about 100,000 residents located near the state capital Sacramento, is part of a series of blazes that have scorched nearly 50,000 acres (20,200 hectares) in northern California in recent days.

Tens of thousands of homes were threatened by flames that drove through dense and bone-dry trees and brush.

Many of the fires were sparked by lightning strikes from brief thunderstorms — nearly 12,000 since last weekend — as a high-pressure area over the West brought a dangerous mix of triple-digit weather and monsoonal moisture pulled from the south.

Henry Wofford, spokesman for the Napa County Sheriff’s Office, told the San Francisco Chronicle that the bodies of three people were found in a home that burned in Napa.

According to the authorities, at least two other people were missing and more than 30 civilians and firefighters have been injured.

The group of fires — known collectively as the LNU Lightning Complex and taking place as the state faces a torrid heat wave — has so far destroyed dozens of buildings or structures in three counties.

On Wednesday, Governor Gavin Newsom declared a state of emergency to facilitate the release of emergency funds.

Nearly 45 million people across the western United States were under an excessive heat warning or heat advisory.

In 2018, of the 44 people killed, 42 died in Northern California’s Camp Fire, while two others were killed in the Woolsey Fire in Malibu.

The fire came amid red flag warnings issued for some areas in Southern California due to extreme wildfire risk.

(With inputs from agency)