The blast at a Pennsylvania chocolate factory in March that left seven people dead was caused by natural gas, the National Transportation Safety Board said in a preliminary report issued Tuesday. 

A “natural gas–fueled explosion and fire” at the R.M. Palmer chocolate company destroyed one building and caused “significant structural damage” to an adjacent building and other nearby structures, according to the NTSB. 

The findings come after the agency said its investigators examined the explosion site, secured evidence and interviewed employees and other witnesses around the time of the blast.  

The NTSB is continuing to investigate the explosion and will now focus on identifying the source of the explosion, it said, as well as reviewing collected evidence, industry practices and federal regulations.

In interviews after the blast, surviving Palmer employees recalled that they “were sanitizing equipment in the building when they detected an odor of natural gas,” the agency said.

Employees in a nearby building that sustained heavy damage “recalled the smell of rotten eggs around the same time,” the NTSB said. 

Natural gas service was provided to the two Palmer buildings by UGI Utilities “through two natural gas mains adjacent to the accident site,” the report said. The utility company said there was “no known work in the area and no pressure spike in gas usage before the explosion,” according to the NTSB. 

The utility company said in the statement Tuesday that it remains “focused on providing accurate, timely and comprehensive information to the NTSB to assist them in analyzing and understanding the events of that day.”

The R.M. Palmer Company did not immediately respond to a request for comment on the preliminary report.  

In a previous statement, the company said it remained “devastated by this tragedy and by the loss of our friends and colleagues.”

R.M. Palmer said in that statement that because it was asked to assist in the federal investigation, “under applicable federal regulations, we are precluded from commenting on anything related to the ongoing investigation, including any allegations that may be made in litigation relating to the incident.”

“Our employees’ safety and health has always been, and will continue to be, of paramount importance,” the company said.

Seven people were killed and 11 others were injured when an explosion rocked the facility in West Reading, eastern Pennsylvania, on March 24.

Authorities identified those who died as: Michael D. Breedy, 62; Diana M. Cedeno, 44; Domingo Cruz, 60; Susan H. Halvonik, 63; Judith Lopez-Moran, 55; Xiorky D. Nunez, 30 and Amy S. Sandoe, 49.

Relatives and loved ones of the victims have been demanding answers about the deadly blast. 

Cedeno was a “total sweetheart” who became a caretaker to her siblings after their mother died, her brother Frankie Gonzalez previously told NBC News. Cedeno was also devoted to her two children, he said, and she and her husband both worked hard to provide for their family.

Gonzalez said he was seeking accountability from the R.M. Palmer Company after reports that workers at the facility complained of smelling gas earlier in the day. He said that two other family members had also previously smelled gas at the factory and had since quit.

Gonzalez said he is waiting for answers on what caused the deadly blast.

“No amount of money is ever going to bring my sister back,” he said. “I lost my heart. My sister wasn’t collateral damage.”