Fire officials say a preliminary investigation has found a nitric acid reaction caused an explosion at the New Hampshire Ball Bearings factory in Peterborough on Monday.
State and local fire officials say they will continue to investigate the blast, which injured 15 people, including two seriously.
The on-scene portion of the investigation was completed Tuesday evening, but a final report is still likely several weeks away.
The incident continues to be classified as an industrial accident.
Nitric acid is an oxidizing agent and is commonly used in industrial settings.
An investigative team will continue to conduct interviews and analyze evidence from the scene.
The company kept employees home again on Wednesday with the goal of re-opening certain sections of the building Thursday.