Verizon may have violated Maryland state law when it did not notify emergency call centers in Montgomery and Prince George’s counties that it experienced network failures several times over the past year, outages that prevented thousands of 911 calls from going through, according to the Maryland Public Service Commission.

The commission, which has been investigating 911 outages in the state, found that in four separate incidents dating to last July, large numbers of callers seeking emergency dispatchers got busy signals. The most high-profile incident of blocked calls was during the Jan. 26 snowstorm that stranded commuters for hours. That day, with traffic accidents occurring across the region and many power lines down, about 8,300 emergency calls in Montgomery county and 1,700 calls in Prince George’s county did not go through.

The problems affected calls from cellphones trying to reach the 911 centers through Verizon lines. The company is the sole provider of 911 services in Maryland.

County officials told the commission that they never received notification from Verizon about the outages. If they had known, county officials said, they could have provided an alternate 10-digit emergency number or alerted the public to use land lines.

Such outages, they said, can be catastrophic.

“Is somebody having a heart attack? Is it a victim of a crime calling?” said Charlynn Flaherty, director of Prince George’s Public Safety Communications. “If they can’t get through to us, then we can’t send them the help they need. And that’s what 911 is all about.”

The five-person commission made an initial finding that Verizon violated a section of state law that requires utilities to provide “equipment, services and facilities that are safe, adequate . . . and efficient,” it wrote in a March 25 order. “Verizon’s lack of prompt and timely notification to the [call centers] that some or all of the 911 trunks were not working properly and calls were not being delivered during these emergency situations is unacceptable.”

The PSC could impose fines of up to $10,000 for each of the outages. Before making a final decision, the commission has ordered Verizon officials to appear Tuesday, giving the company the opportunity to show that it did not break the law and should not receive civil penalties.

The Federal Communications Commission, which regulates 911 and has the authority to impose federal fines, also is looking into Verizon’s network outages.

Aside from the Jan. 26 snowstorm, Montgomery and Prince George’s counties had 911 problems last year on July 25 and Dec. 17, and then again on Jan. 31. The numbers of blocked calls for those dates were not publicly available.

“We are determined to hold Verizon accountable for its performance in this critical area,” said Patrick Lacefield, a spokesman for Montgomery County. “At the same time, we are exploring our own backup alarm system so we don’t have to rely solely on Verizon.”

Verizon has said that the call problems were related to high volume and that the company has taken steps to improve service.

“When issues arise, we work diligently to resolve them, investigate their cause and take measures to prevent their recurrence,” Sandra Arnette, a Verizon spokeswoman, said in an e-mail. “We will continue to work with the 911 centers and others to ensure that callers receive the level of service they deserve and expect when they dial 911.”

The main complaint from local officials is that Verizon never notified them when the emergency phone lines went down, even though Verizon is supposed to be continually monitoring its networks.

According to the PSC’s investigation, the Verizon operations center was aware of the outages but never notified its “customer care center,” the Verizon office that works directly with local 911 centers. Verizon officials told Maryland commissioners that their operations center is not required to report problems with emergency telephone lines and equipment.

In February, the FCC asked Verizon to explain what happened with 911 calls during the Jan. 26 storm.

“The public rightly expects that they can use 911 to reach the appropriate first responders in an emergency,” Jamie Barnett, chief of the FCC’s public safety and homeland security bureau, wrote in a letter dated Feb. 17. “We are particularly concerned that this problem may be widespread across Verizon’s footprint.”

Verizon responded last month that it will now notify county emergency centers of phone trouble within 15 minutes. FCC spokesman Robert Kenny said the commission has asked Verizon for more information and could not comment further on its investigation.