Private investigators, the U.S. Chemical Safety and Hazard Investigation Board, and Region 5 of the U.S. Occupational Safety and Health Administration will continue their probes of the incident.
The Waukegan Fire Department's investigation was hampered because the key witnesses died in the explosion, according to Waukegan Fire Marshal Steve Lenzi.
Byron Biehn, 53, a second-shift production supervisor and company owner;
Allen Stevens, 29, an owner and second-shift chemical operator;
Jeff Cummings, 57, a third-shift production supervisor; and
Daniel Nicklas, 24, a quality control chemist.
Stevens died in the hospital shortly after the explosion. Biehn and Cummings were found in a search of the wreckage May 5, and Nicklas was found the following day.
Officials of AB Specialty Silicones did not return multiple telephone calls.
The company, however, did issued the following statement on its website in June: "While AB Specialty Silicones continues to process and heal from the tragic event that occurred on May 5, 2019, we want to reassure our customers and partners that we are open and have resumed operations.
"We will work with each individual customer to ensure we meet the high-quality service in which ABSS prides itself."
On May 31, Illinois Attorney General Kwame Raoul and State's Attorney Michael G. Nerheim filed a lawsuit in the Lake County, Ill., District Court.
The suit alleged that the explosion at AB caused an unknown amount of pollution to the land and water near the facility, and sought to require the company to perform an environmental cleanup.