As wind-fanned flames raced across several counties in Northern California, only one activated the government’s most potent public notification system — a blast that overrides the volume controls on cellphones, turning them into the equivalent of squawking alarms.

The simultaneous wake-up call for many of Lake County’s 64,000 residents came shortly after 2 a.m. Monday, after county emergency officials decided to order a mandatory evacuation because of the inferno known as the Sulphur fire.

“We decided it was the right course,” Lt. Corey Paulich, spokesman for the Lake County Sheriff’s Office, said of the alert. “We felt that our residents were in imminent danger.”

In neighboring Sonoma County, however, officials decided not to send an alert because of concern that it might cause panic and clog roadways, potentially blocking rescue workers or even leaving thousands exposed to shifting, wind-driven fires.

The fires did not treat the nine affected counties equally, and the destruction in population centers was worse in Sonoma than anywhere else. But all 36 confirmed fire-related fatalities were in Sonoma and three other counties that did not send wireless alerts. No deaths have been reported in Lake County.

Sue Fellbaum returns to her home of 28 years that has been burned to the ground by the wildfires in Santa Rosa, Calif. (Whitney Shefte,Elyse Samuels/The Washington Post)

Five years after it was launched by the Federal Emergency Management Agency, the use of the nation’s alert system remains uneven. And despite a campaign by FEMA to encourage local governments to participate, most U.S. counties could not order an alert today if they faced an emergency. More than 65 percent of the nation’s 3,500 counties do not have agreements in place with FEMA to send alerts through the Wireless Emergency Alert system, as it is known, the agency said.

The alerts are sent to all phones in the targeted area, often accompanied by a vibration and a unique sound that FEMA says is “designed to get your attention.” The majority of alerts sent over the system since 2012 — more than 25,000 — have been related to flash floods, tornadoes and other weather events, FEMA records show.

There have been missteps. Authorities were criticized in Southern California in June when an alert was erroneously sent to a wide swath of Riverside County, which has a population of more than 2 million, directing residents to “evacuate now” because of a wildfire. Officials took to social media to stress that the evacuation was only for a small area of the county.

But a trade group for wireless carriers credits the alerts with helping to save lives during extreme weather events and other emergencies, including a tornado in Connecticut and child abductions in many states.

Of the four counties in Northern California where residents were killed in fires this week, two — Sonoma and Mendocino — had agreements in place with FEMA that enabled them to send alerts. Yuba and Napa counties did not, according to federal records.

That left Napa residents like Emily Cocks with no warning before fire appeared in view of her home. Cocks said that her husband woke up around 12:45 a.m. Monday after he heard branches hitting their bedroom window from high winds in the Atlas Peak area. Looking outside, the couple saw fire on the ridge above them. They packed their cats, some clothing and Cocks’s grandmother’s jewelry, then left for her parents’ home 100 miles to the south. Before they arrived, on a news report, Cocks’s father spotted the home in Atlas Peak engulfed in flames.

Of the lack of an alert, she said, “In hindsight, it would have been nice.”

Smoke lingers in the air as the sun rises over firefighters’ staging area at the Sonoma County Fairgrounds. (Stuart Palley/For The Washington Post)

Molly Rattigan, Napa’s deputy county executive officer, said the county has long opted to do evacuations in person. “We’re a small community and going out to people in person, and helping them, that’s been what’s worked well for us,” she said.

Napa used its Nixle service Monday night, a system that allows for a kind of reverse 911 calling and emergency text messaging to those who sign up voluntarily. Two people in the county died Monday; two more deaths were confirmed Friday. Before the fire, 20,000 people in the county of about 140,000 had signed up for the service, she said. Since Monday, the number has grown to 135,000.

Officials in Yuba County defended the decision not to rely on the alerts system. Cell service is spotty in the rural area, said Russ Brown, a spokesman for the county’s office of emergency services. Instead, Yuba used a reverse 911 system dubbed CodeRED, along with old-fashioned bullhorns and sirens, to wake people, Brown said.

Brown dismissed the idea that a wireless alert would have prevented the four deaths from this week’s fires in the county, saying that cellphone towers were among the first structures overwhelmed by flames.

Emergency officials in Sonoma County, where fires incinerated whole neighborhoods in the city of Santa Rosa, said at a news conference Wednesday that the dwindling number of landlines added to the challenge of reaching people through reverse 911 calls.

Sheriff Rob Giordano urged Sonoma County residents to sign up for its voluntary system of emergency alerts — which, like Napa, Sonoma used to spread evacuation orders. As of earlier this summer, only 10,000 of the county’s 500,000 residents had signed up.

On Thursday, Sonoma County emergency officials offered a different explanation. They said they had consciously decided not to use the FEMA-backed wireless technology for fear of causing mass panic and “because the warning is not targeted,” said Jennifer Larocque, a county spokeswoman. But WEA notifications can and have been targeted to smaller geographic areas, FEMA said Friday.

The agency pointed to the wireless alert issued Monday at the request of Lake County officials. That alert went to a six-mile-wide swath of residents in and around the city of Clear Lake, according to a map provided by FEMA.

Among those who received the warning was Josy Range, who noticed a vague smell of smoke when she woke up unexpectedly around 2 a.m. Almost immediately, Range said, her cellphone erupted with messages, including one that was delivered with a sharp ring unlike an ordinary text message. “It sort of sounded like a siren,” she recalled.

The message read: “Mandatory evacuations for Sulpher Fire. Elem Colony & N. Clearlake Shelter ­@ Twin Pine,” according to FEMA.

Range heeded the warning. She got dressed, grabbed her medications and her three dogs, and climbed in her car. She soon hit a roadblock set up by the fire department, where other neighbors had gathered to gaze at the glowing spectacle that had materialized, seemingly within minutes, on the ridge above her home.

“I looked back at my house and I could see a row of flames on the hillside above it,” recalled Range, 65, who at that point pulled out her cellphone once more. “I said, I guess I should take a picture of this.”

Lisa Bonos contributed to this report.