A massive blast at a Tennessee explosives plant that killed 16 people, leveled the building and was felt more than 20 miles away began in an area where workers used kettles to produce a mixture of explosives, and set off other explosives stored nearby, authorities said Friday.

Investigators still haven't been able to identify the cause of an Oct. 10 explosion at the Accurate Energetic Systems factory in Bucksnort, an unincorporated community about 60 miles (97 kilometers) southwest of Nashville, officials said at a news conference.

The delicate investigation at the site of the plant ha concluded, said Brice McCracken, the Bureau of Alcohol, Tobacco, Firearms and Explosives’ special agent in charge at the National Center for Explosives Training and Research. In addition to locating victims' remains, it involved removing and disposing of explosives that didn't detonate in the blast.

The next phase will involve work at ATF labs and testing facilities, where investigators will try to determine what triggered the explosion, said Jamey VanVliet, ATF special agent in charge in the Nashville division.

From 24,000 to 28,000 pounds of explosives detonated that day, authorities said. The blast originated on the large plant's first floor, near kettles used in the production of an explosive mixture for the commercial mining industry, McCracken said.

The explosives in question, known as cast boosters, are typically a mixture of TNT and RDX, or cyclonite, that are poured by hand into a cardboard tube, he said.

After the initial explosion happened in those production kettles, investigators believe other explosive materials stored on the main floor also detonated, McCracken said.

During the investigation, authorities searched an area of about 500 acres (200 hectares), much of it dense with woods, looking for evidence.

Although the on-site probe has wrapped up and the scene was turned back over to the company Thursday, it will likely be months before the investigative concludes, McCracken said.

AP Video by Kristin M. Hall