About 2.3 million Northern Virginia residents lost emergency 911 service for up to four days after the June 29 thunderstorm, in part because a backup generator would not start, a senior Verizon official told government leaders Wednesday.

The outage prevented hundreds, perhaps thousands, of calls for help from getting through to emergency responders in Fairfax and Prince William counties, as well as in nearby cities. The storm also cut power to a million customers, took out land-line and wireless phone service, toppled hundreds of trees and forced a boil-water order in Falls Church.

Thirty-one people died as a result of the record heat in the days before and after the storm.

“This region expects better,” Prince William County Supervisor Frank J. Principi (D-Woodbridge) said at the Metropolitan Washington Council of Governments board meeting. “What is particularly concerning to me is it’s not the first time this has happened.”

Investigations are underway by both the Virginia State Corporation Commission and the Federal Communications Commission, as well as by Verizon.

David Turetsky, the FCC’s bureau chief for public safety and homeland security, said his agency is looking into the outage as well as those that occurred in the January 2010 and 2011 blizzards, and during last summer’s Hurricane Irene.

“We intend to get at the facts,” he said. “This is not idle curiosity. We intend to use the results to make the public safer.”

Added FCC spokeswoman Lauren Kravetz: “It’s hard to say that someone who needed to reach 911 and couldn’t is overreacting.”

The 911 failure began hours after the severe “derecho” passed through the region about 10:30 p.m. June 29. The thunderstorm damaged “multiple points” in Verizon’s network, said Kyle Malady, Verizon’s senior vice president for global network engineering and operations, who is leading the company’s investigation.

“The magnitude of this event cannot be overstated,” he said.

When the power went out, Verizon’s Arlington County hub for Northern Virginia’s 911 service failed.

That hub funnels 911 calls to the proper jurisdiction. The facility reverted to a battery backup, Malady said. Twin diesel-powered generators are then supposed to quickly kick in, but one of them could not be started.

Both generators underwent routine testing three days earlier, he told the Council of Governments.

Verizon spokesman Harry Mitchell said in a later interview that the generator failure was only “one factor. We continue to investigate mechanical and equipment problems.”

In the hours after the storm, Fairfax County’s emergency services operations center received what director Steve Souder called “a muted, cryptic notification” that something was wrong in Arlington.

Fairfax officials, who had been dealing with a surge in emergency calls as a result of the storm, didn’t know that the note applied to them. In fact, about 4 a.m., Fairfax shut down its storm response center because officials believed all its calls had been resolved.

By 7:30 a.m., 911 calls had stopped coming in. Police and fire officials could not call out on their emergency phone lines. But Verizon did not notify Fairfax of the failure of the 911 hub until 9:30 a.m., Souder said. Local officials, quickly putting together the extent of the outage, scrambled to get word to the community that the 911 system was not reliable and that residents who couldn’t get through should call non-emergency lines.

Getting through on those lines was spotty at times.

The same storm that damaged the 911 hub also took out traditional landline phones and wireless capacity. Some people relying on FiOS or cable connections for their wireless phones couldn’t make or receive calls. Text messaging wasn’t reliable, either, so local officials contacted radio, television, newspaper, blogs and social media sites, and told residents that if they had an emergency and couldn’t call for help, they should flag down a passing police car or walk to the nearest fire station for help.

“That’s completely ludicrous,” Sharon Bulova, chairman of the Fairfax County Board of Supervisors, said she thought even as she gave the advice. “That’s exactly the time when you need people to come to you.”

“911 is the most recognized phone number in America,” Souder added. “Two hundred and forty thousand times a day, someone calls that number. It absolutely must work flawlessly.”

Mary Pat Flaherty contributed to this report.