Fairfax County’s 911 emergency center operated at just half capacity Monday as Verizon struggled to figure out why both its primary and backup power systems failed after Friday night’s storm and left much of Northern Virginia without 911 service through the weekend.

Callers with medical and safety emergencies caused by soaring temperatures, power outages and downed electric cables received either rapid busy signals, recorded messages saying the line was inoperative or dead silence, even after Verizon’s service was restored, local officials said.

The loss of power from both primary and backup systems, according to Harry J. Mitchell, Verizon’s director of public relations, damaged the company’s computer hardware and software and caused other mechanical problems in a chain reaction that has perplexed and alarmed state and local governments.

“It is understandable that something like this could happen, but shouldn’t there be some redundancy or backup to keep 911 up and running?” said Sharon Bulova (D), chairman of the Fairfax County Board of Supervisors. “It’s not acceptable for the region’s 911 system to go down.”

Bulova and other elected officials are asking the Metropolitan Washington Council of Governments to set up an investigative task force.

Corey A. Stewart (R), chairman of the Prince William Board of County Supervisors, called the 911 outage “shocking” and “unacceptable.”

“I just hope no one lost their lives because the 911 system was down,” he said.

Mitchell said Verizon is restoring 911 service piece by piece.

“Once we complete our restoral efforts, we will investigate fully the causes of the problems and provide a root-cause analysis to the appropriate officials,” he said in an e-mail. “The powerful storm appears to have caused problems on multiple layers of facilities, from the commercial power failure to damage to our backup power supply, to downed and damaged lines. The combination of those factors led to issues with various aspects of the 911 system.”

Regular 911 service was restored Monday in Manassas, he said, and Verizon was again beginning to successfully provide the addresses of all 911 callers to Fairfax County. “We are testing that with Fairfax County now,” he said.

When the system is functioning properly, computers route all incoming calls to authorities in proper jurisdictions with callers’ addresses. If problems are experienced routing calls to one jurisdiction, the system is then supposed to route them to neighboring ones. But this function also failed over the weekend in most of Northern Virginia, except Alexandria and Loudoun County, which are serviced by different networks.

So far, no one has died because of the lack of 911 service, thanks in part to the extraordinary efforts that local governments made to alert residents by radio, television, Web sites, Facebook, Twitter and neighborhood e-mail lists, local officials said.

If residents couldn’t get through on 911, each government provided non-emergency numbers. If those didn't work, residents were advised to flag down a police officer or firefighter, or walk to the nearest fire station for help. Several dozen people in Fairfax stopped by or called fire stations, the majority for non-emergency matters, Dan Schmidt, a Fairfax fire department spokesman, said.

The 911 outage began late Friday, after the rain and windstorm swept through the area. Calls to local 911 centers in Fairfax, Arlington County, Prince William, Falls Church, Manassas and Manassas Park trickled to a stop. Verizon notified emergency responders about 6 a.m. that the power outage that shut down so much of the region had also affected its Arlington-based facility, and its backup system had also failed.

On Monday, 911 service was improved but still wasn’t back to normal, local officials said. For a while, 911 operators could not making outgoing calls. Regular land lines and electronic communications, such as e-mail, were not as reliable as they normally are.

“I’m sorry to say we still are not fully restored,” said Steve Souder, director of Fairfax’s 911 Center. “We are getting some 911 calls, at half the capacity of what it normally would be.”

Jack Brown, director of Arlington’s Office of Emergency Management, who was worried about the combination of heat, lack of power for air-conditioning, non-operating traffic signals and erratic 911 communication, suggested canceling Fourth of July celebrations.

Terrie Suit, Virginia’s secretary of Veterans Affairs and Homeland Security, said she will organize a working group of state and local officials, telecommunication and cable companies to address repeated problems statewide with 911 centers during extreme weather events.

“We need a long-term fix,” said Suit, who jumped into the issue at the request of Virginia Gov. Robert F. McDonnell (R). “We’ve had 911 issues during hurricanes, during Irene. . . . We’re all impacted when there’s a major storm, no matter who the vendor is. We’ve got to get to the bottom of this.”

In 2011, Maryland’s Public Service Commission criticized Verizon for failing to inform communities when 911 calls failed to go through. At the time, Verizon pledged to alert county 911 centers of phone problems within 15 minutes.

Apart from the 911 outage, cellphone service across the Washington area remained spotty Monday.

AT&T, Sprint, T-Mobile and Verizon all reported that they are working to get generators to sites that have lost power and repairing storm damage as necessary. Customers have reported being unable to make phone calls and, in some cases, also reported text message outages. AT&T and Sprint said technicians were working long hours to restore power and conduct repairs. T-Mobile said Monday that about 85 percent of the network’s cell sites were running.

Comcast and Cox Communications both said their companies can’t completely restore service until customers have power and can determine whether their cable and Internet connections are also damaged. Fallen trees and branches have knocked out some cables across the region. Those issues will take longer to repair.

Brookland resident Tony Taylor’s phone dropped half a dozen calls as he made his way out to Bowie to check on some property. Though cell service was spotty Saturday and Sunday, he couldn’t make any calls or use text messages for several hours.

The outage forced Taylor, who stopped land-line service in 2008, to rely on backup prepaid phones to conduct business.

“For the last several years, I’ve been used to having a cellphone on the go,” he said.

According to a 2011 study from the Centers for Disease Control and Prevention, nearly one-third of all American homes no longer have a landline.

The Federal Communications Commission works with carriers on best practices for 911 call center operations for such issues as taking calls from Skype or similar voice-over IP services.

But 911 centers work with states on how to reroute calls in case of outages. If companies do not adhere to these best practices, the agency can put out a public notice asking carriers to adhere to their agreements more closely.

In cases such as this, the agency analyzes data it receives from the carriers to determine if they need to follow up on any issues.

After Hurricane Katrina, the FCC tried to make a regulation that would require telecommunications companies to provide enough backup power for eight hours at all cell sites. After a lawsuit, the regulation was struck down when the government ruled that the FCC did not provide enough public comment on the law. The agency has since suggested reopening the issue.

Catherine Caton of Clifton said she is furious with Verizon, her telecommunciation provider, which won’t give her a date when it expects to fully restore her home phone service.

“It confounds me that they are held to a different standard,” she said. “They have an obligation to provide service.”

The 911 outage delayed Prince William paramedics from getting to a person suffering from a cardiac issue over the weekend, said Jason D. Grant, a county spokesman. Grant said the caller reporting the emergency got a busy signal initially but eventually got through.

Instead of the normal three- to five-minute response, it took emergency crews about 10 minutes to get to the patient, Grant said. The patient had a pulse upon arrival at the hospital, Grant said. Additional details about the patient’s condition and the incident were not immediately available.

Jeremy Borden, Caitlin Gibson, Justin Jouvenal, Cecilia Kang and Hayley Tsukayama contributed to this report.