Federal investigators have discovered part of the cause of a 2016 explosion at a Silver Spring apartment complex that killed seven people and injured dozens more, several outlets have reported. Investigators announced their findings at a National Transportation Safety Board meeting on Tuesday.

According to NTSB, which has been trying to determine the cause of the fire for almost three years now, an unconnected gas vent line meant to be attached to a mercury service regulator allowed natural gas to leak into the basement room, WTOP reports. The vent was owned by Washington Gas, per WTOP. The gas was ignited by an unknown ignition source on August 10, 2016, causing an explosion that destroyed the Flower Branch Apartments.

The explosion and resulting fire tore through the building, and had a devastating impact on the families who lived there. Authorities found seven residents dead in the debris over the course of several days after the explosion. Thirty people were injured and 84 families were displaced. Families eventually filed lawsuits against Washington Gas and Kay Management, the apartment building’s management company. The residents of the Flower Branch Apartments were mostly low-income Spanish-speaking immigrants.

Washington Gas was responsible for maintaining the vent line that caused the explosion, and the utility had previously suggested that the line had disconnected during the explosion, WTOP reports. But NTSB investigators said it had probably been disconnected for a while before the explosion, according to the outlet.

The Bureau of Alcohol, Tobacco, and Firearms had also previously said the probable cause of the explosion was a natural gas leak. But the NTSB’s findings contained new details outlining several failures to protect residents in the weeks leading up to the explosion.

Residents reported gas smells to Kay Management six times in the weeks and months leading up to the August 10 explosion, NTSB investigators announced on Tuesday. Maintenance staff for the management company reportedly said each time that they didn’t detect any gas, or else they attributed the smell to freshly painted apartments.

Two weeks before the explosion happened, a resident called 911 to report a gas smell, officials said. But Kay Management had recently changed the lock on the basement meter room without giving the fire department a new key, the Washington Post reports. Firefighters pried the door partially open, but didn’t detect any gas.

On the night of the explosion, the same resident who called 911 again smelled gas, getting stronger as he moved toward the basement, WTOP reports. But before he could again call 911, the room exploded.

The NTSB’s investigation has been ongoing for three years, frustrating families trying to move forward with lawsuits. A final report from the agency is forthcoming.