FALLS CHURCH, Va. — A gas company's faulty equipment is the most likely cause of an explosion at a suburban Maryland apartment complex that killed seven people in 2016, according to a federal report issued Tuesday.

The explosion and fire at the Flower Branch apartments in Silver Spring caused a partial building collapse and sent 68 people, including three firefighters, to the hospital.

The National Transportation Safety Board, or NTSB, has been investigating the cause of the explosion for nearly three years, and officials there called it one of their most difficult cases because so much evidence was destroyed.

At a meeting Tuesday, the board unanimously adopted the findings of its investigators, who determined that a faulty regulator that had been left unconnected to a vent pipe inside a basement meter room most likely caused the explosion.

Fire and rescue crews and investigators work at the site of an explosion and fire that destroyed a building in the Flower Branch Apartments complex in Silver Spring, Maryland, on Aug. 11, 2016. T.J. Kirkpatrick / Getty Images file

Washington Gas, the company responsible for maintaining the equipment that the NTSB blames for the explosion, has disputed the board's findings. President and Chief Executive Adrian Chapman said in a statement Tuesday that the utility was saddened by the incident and remained committed to safety.

"However, we disagree with their findings as we don't believe the evidence indicates failure of our equipment that night. We also do not believe the NTSB sufficiently investigated the other potential causes of the explosion," Chapman said.

The board's findings do not point a finger just at Washington Gas. The board also found that multiple communication gaps and missed opportunities to correct the leak might have prevented the explosion.

NTSB Chairman Robert Sumwalt said that people around the complex smelled natural gas six times in the weeks and months before the explosion but that nobody called the gas company to report a possible leak.

Most glaringly, a resident called 911 to report the smell of natural gas on July 25, about two weeks before the explosion.

Firefighters responded and could not detect natural gas. But they were thwarted from conducting a thorough investigation because they could not gain access to the meter room from which the leak likely emanated. The locks had recently been changed, and firefighters did not have access to the correct key, even though local code requires that firefighters have such access.

Because no leak was detected, firefighters did not call Washington Gas. Neither did the 911 dispatcher.

Sumwalt: Pipelines provide heating and cooking fuel to millions of homes and businesses throughout the nation. But when pipelines fail, the results can be tragic—as they were the night of the explosion and fire in Silver Spring. — NTSB_Newsroom (@NTSB_Newsroom) April 23, 2019

The NTSB is recommending that 911 dispatchers automatically call the gas company any time a person calls to report the smell of natural gas, rather than rely on the public to make the call.

"We missed a very good opportunity to possibly have stopped this whole thing," NTSB Vice Chairman Bruce Landsberg said.

Among the other recommendations: to onstall regulators outside rather than inside so any venting occurs safely into the atmosphere and to expedite phaseout of mercury-based regulators like those that were in place at the apartment complex.

Most of those regulators, which date to the 1940s and the 1950s, have been targeted for phaseout because of environmental concerns over mercury. But NTSB staff said the age of the equipment increased the likelihood that they would fail and cause gas leaks.

The NTSB took the lead in the investigation because it has jurisdiction over gas pipeline accidents.