An ambulance makes a run at the Bethesda-Chevy Chase Rescue Squad. (Jahi Chikwendiu/The Washington Post)

A series of mishaps and systemic weaknesses, starting with a cooling unit low on refrigerant, caused Montgomery County’s 911 system to crash for nearly two hours on the night of July 10, causing about 100 callers to get busy signals when they tried to report emergencies, officials said Tuesday.

Delays caused by the collapse may have been a factor that evening in the deaths of a 40-year-old dialysis patient in Twinbrook and a 91-year-old Olney woman. Montgomery officials said they need to collect more data before they can make a determination.

Senior officials were grilled by County Council members, who expressed alarm at the failure of local government’s most essential service. Council member Nancy Navarro (D-Silver Spring) said that if she had “a recurring nightmare . . . this would be the one.”

Officials made no excuses.

“That should never happen, but it did, and now our objective is to make sure it never happens again,” said Chief Administrative Officer Timothy L. Firestine.

He said the 911 system had been scheduled for an internal audit later this year because of its “high-risk” status — meaning it is a vital system and a service lapse could have serious consequences.

The 911 operation was undergoing an overhaul before it failed. On June 22, the county’s Emergency Communications Center in Gaithersburg was moved to its alternate site in Rockville while phones and other systems were upgraded.

Initial signs of trouble surfaced July 2, according to the narrative presented by officials. That is when a system that collects data from county buildings — including the performance of heating and cooling systems — began to suffer “intermittent outages.” Workers found no issues, but on July 8, a new server was installed. A day later, it again showed similar problems.

Sometime on July 10, the HVAC unit that cooled the room housing the power source for batteries at the Rockville communications center failed, causing temperatures to rise past 120 degrees. At 9:13 p.m., the system — to protect itself — went into “bypass” and switched to PEPCO power from lines outside the building.

At 11:08 p.m., what the county called an unexplained “anomaly” in Pepco service caused power to the 911 system to flicker. It didn’t last long enough to trigger emergency generators, but it essentially froze the entire system, officials said.

The county’s 36 emergency phone lines were down from 11:08 p.m. to 1:09 a.m. on July 11.

David Dise, the county’s director of general services, said the HVAC unit was more than 20 years old and near the end of its service life. But routine maintenance had been performed in May, he said, and no coolant leak was found.

Dise said the county was in the process of adding a backup HVAC unit.

Council members said the entire system needs a thorough review.

“This is really complex, and a lot of dominoes fell in a way that people just didn’t expect,” said Marc Elrich (D-At Large), chairman of the Public Safety Committee.

Members were especially concerned that it took until 12:24 a.m., more than hour into the outage, for the “Alert Montgomery” system, which sends updates and warnings to residents, to be activated.

But Roger Berliner (D-Bethesda), said that even timely alerts would not be useful. He said that if he were in the midst of a life-threatening emergency, “Would I be looking at my cellphone for alerts? I don’t think so.”

Police officials said residents should be encouraged to keep phone numbers of their local police and fire stations handy, but council members dismissed the idea as impractical, saying numbers stuck to a refrigerator door would not necessarily be handy in an emergency.

County officials said they will submit a final report on the incident to the council in the fall.

Verizon, the county’s telecom provider, played no role in the outage, officials said.

That has not been the case in other areas that have suffered 911 collapses.

Earlier this month, GCI, an Alaska company, agreed to pay a $2.4 million federal fine for a series of 911 phone-service outages beginning in 2008. Last year, the FCC fined T-Mobile $17.5 million for problems that interfered with its customers’ emergency calls.