Officials have not named the four people who were killed in the shooting, or the eight others who were injured, including one who was in critical condition.
Michigan Governor Gretchen Whitmer urged residents not to speculate about the motive as officials conducted their investigation.
"Speculation is unhelpful and can be downright dangerous, so I just ask that people lower the temperature of the rhetoric," she said.
The shooting began at around 10:30 EST (15:30 BST) on Sunday, when a gunman crashed his vehicle into the building and then began shooting at congregants.
One man told CBS News that he and others went outside the church after hearing a "loud boom" of the vehicle crash, before seeing the suspect begin to fire a weapon at the building.
"It was just so unexpected," said the congregant, Paul Kirby. "Once I saw the gun and start hearing him start shooting it, it was just a lot of fear."
Soon after, the shooter set the church ablaze using gasoline or another accelerant, officials said. They also found the suspect's makeshift explosive devices, said James Deir, an agent of the Bureau of Alcohol, Tobacco, Firearms and Explosives.
Less than 10 minutes after the shooting, the gunman died in a shootout with police, officials said.
Investigators said on Monday that they had interviewed over 100 victims and witnesses as a part of the investigation and spent the whole night processing the scene of the crime.
[SRC] https://www.bbc.co.uk/news/articles/c62n45v0rz2o
