— Verizon executives and leaders of Northern Virginia’s 911 emergency centers agree that a brutal storm passed through the area June 29 and that, for a while, people couldn’t reach police, fire and other emergency responders by dialing 911. They agree that a breakdown in Verizon’s network prevented calls from getting through. They also agree on the importance of 911 for public safety.

They disagree on almost everything else.

Verizon officials told the Virginia E-911 Services Board in suburban Richmond on Thursday that only four counties or towns in Northern Virginia lost 911 service; Fairfax County’s emergency services director said at least six were without reliable emergency service.

Verizon said 911 calling service was restored in Fairfax and Prince William counties by 2:40 p.m. June 30. Local 911 officials said it wasn’t until July 3 that the service was fully reliable, including displaying callers’ addresses.

About Arlington, they are completely at odds.

“The Arlington [emergency center] never lost 911,” said Maureen Davis, the utility’s vice president of network operations.

“That’s not true,” said J.D. Crawford, commander of Arlington County’s emergency communications center. “We had intermittent problems both with 911 and non-emergency calls . . . [through] July 2.”

Information about how the outage affected millions of Northern Virginians continues to dribble out as Verizon officials and local 911 chiefs try to explain how the outage happened and why it took so long to get it fixed. About a dozen Verizon officials attended the Metropolitan Washington Council of Governments meeting Wednesday, and most of them also traveled to the Virginia E-911 Services Board meeting.

The Federal Communications Commission and the Virginia State Corporation Commission have announced plans to investigate the outage, and Verizon appointed Kyle Malady, its senior vice president for global network engineering and operations, to lead its probe.

Emergency services leaders appeared surprised Thursday that a system as important as delivering 911 calls has only one “plan B” if power goes out.

“You have a single point of failure — your generator?” asked Sam Nixon, an E-911 Services board member and chief information officer of the commonwealth. Verizon officials replied that the single set of paired generators is the sole backup when commercial power goes out.

There were 168 local response centers for 911 calls in the path of the “derecho” storm in the greater Washington area, Davis noted, and by Verizon’s count, 164 did not lose power. When another board member asked whether Verizon would test generators in other areas of the state, Davis said, “I actually think the storm itself gave us a very good test.”

After the brief but furious storm struck at 10:30 p.m. June 29, Verizon found “multiple and extensive damage” to its facilities and phone lines in the region.

Just before 11 p.m., power went out at a critical central office in Arlington that serves Fairfax. Batteries kicked in to keep phone calls moving to the proper 911 call center, but the batteries last for only about eight hours.

Technicians arrived at the facility in the Clarendon neighborhood shortly after midnight, Davis said, and began trying to start two generators that together act as the backup power source. But one of the generators, which had worked smoothly in a routine test three days before, would not start. When the batteries drained, Verizon was unable to deliver 911 calls from the heavily populated urban and suburban areas. The local 911 call centers noticed quickly that they were no longer receiving emergency calls.

Verizon officials said the outage affected Fairfax, Prince William, Manassas and Manassas Park — about 1.5 million people.

But local leaders said it also stopped calls from Arlington, parts of Alexandria and other areas of the region, which would increase the population affected to 2.3 million.

“You just have to have reliable backup,” said Steve Souder, Fairfax’s emergency services director. “This was a major event. . . . I was not able to say we had reliable, dependable 911 service until 3 p.m. Tuesday, July 3.”