It was the second explosion at Independence Professional Fireworks near Osseo in less than four months.
Victims include Robert and Patricia Slayton, co-owners of the factory.
Robert Slayton was badly burned. He died at a Kalamazoo hospital Monday night.
Investigators with the federal Bureau of Alcohol, Tobacco and Firearms will be back at the site Tuesday.
The December explosion killed seven people. Its cause has never been determined.
Osseo is about 80 miles southwest of Detroit and 15 miles north of the Ohio state line.
The December explosion killed six women and one man while injuring 13 others. Federal, state, and local investigators determined that blast occurred in a shell assembly room, leaving only a concrete slab with debris and human remains scattered over a wide area.
Their report found no indications of criminal activity, and investigators have said they may never fully understand what caused that blast.
Independence is among the country's largest makers of display fireworks shells.
In an average year, the company provides fireworks for up to 65 Fourth of July celebrations. The company also provides fireworks for other events throughout the year, according to a July 1996 article by The Jackson Citizen Patriot.
Independence makes all of its products by hand, using materials like nitrates, sulfurs, charcoal and black powder. Because of the dangerous materials, the company has a number of precautionary measures in place, the newspaper said.
Since the December explosion, the factory was free to resume its operations, said Maura Campbell, spokeswoman for the Department of Consumer and Industry Services, which licenses fireworks manufacturers.
The state did not revoke or suspend the company's license after the December explosion, pending the outcome of its investigation.