Investigators were focusing on what motivated a former Marine to ram a pickup truck into the sanctuary of a Church of Jesus Christ of Latter-day Saints in Michigan and open fire during a crowded service, killing at least four people while setting the building ablaze.

Crews continued searching for victims in the charred rubble Monday as authorities said “some” people were unaccounted for following the Sunday morning attack that wounded eight others in Grand Blanc Township, about 60 miles (96 kilometers) north of Detroit.

The FBI considered the attack — the second on an American church in little over a month — an “act of targeted violence,” said Ruben Coleman, a special agent in charge for the bureau.

Authorities identified the shooter as Thomas Jacob Sanford, 40, of the neighboring town of Burton. Investigators deployed a robot while searching Sanford’s residence Sunday but did not say what they found or provide any additional details about him, including whether he had any connection to The Church of Jesus Christ of Latter-day Saints, widely known as the Mormon church.

AP video shot by Mark Vancleave and Isabella Volmert