The search for more potential victims of a mass shooting at a Michigan church continued into Monday, after a gunman killed at least four people while hundreds were worshiping on Sunday morning.

Gunman Thomas Jacob Sanford, 40, drove his truck into The Church of Jesus Christ of Latter-day Saints in Grand Blanc, Michigan, on Sunday morning, before shooting congregants and setting the building on fire, according to officials.

At least four people were killed and eight others were injured in the shooting and fire, according to police. The gunman was then killed in a shootout with responding police, law enforcement said.

The FBI is leading the investigation, calling the attack an “act of targeted violence.”

Officials confirmed that of those shot, one victim died at the scene, another later died at the hospital and two more individuals were found dead at the scene due to the fire. Eight others remain hospitalized, seven are in stable condition and one is in critical condition.

Grand Blanc Township Police Chief William Renye told reporters Sunday, “We do believe we will find additional victims once we have that scene secured.”

Officials said that the chapel is a “total loss” as investigators work to comb through the rubble.

A source briefed on the investigation told ABC News that detectives are urgently working to determine the motive behind the shooting. 

Investigators are working to learn whether the church had been the target of threats in recent months and looking to see whether the timing could be connected to the death on Saturday of Russell M. Nelson, president of The Church of Jesus Christ of Latter-day Saints. He was 101 years old.

Renye said during Sunday’s news conference that the FBI had assigned more than 100 agents to help in the investigation.

Renye said the gunman “ran the vehicle through the front door, exited and started firing shots,” adding that it remains unclear what connection, if any, the suspect had to the church.

Sanford was a veteran of the Iraq War, according to officials. ABC News confirmed with the United States Marine Corps that Sanford served four years in the Marines from June 2004 to June 2008. He ultimately rose to the rank of sergeant, officials said, serving one combat tour to Iraq.

President Donald Trump said had been briefed on the shooting and fire, writing Sunday on social media, “This appears to be yet another targeted attack on Christians in the United States of America.”

“The Trump Administration will keep the Public posted, as we always do. In the meantime, PRAY for the victims, and their families. THIS EPIDEMIC OF VIOLENCE IN OUR COUNTRY MUST END, IMMEDIATELY!” Trump said.

Trump also wrote that the FBI is leading the investigation efforts. Trump said that while the suspect is dead, there is “still a lot to learn.”

Vice President JD Vance posted his own statement on social media, calling the shooting and fire at an LDS church “awful.” He said the “entire” Trump administration is monitoring the incident.

Michigan Gov. Gretchen Whitmer said her “heart is breaking for the Grand Blanc community” in a statement on X.

“Violence anywhere, especially in a place of worship, is unacceptable,” she said. “I am grateful to the first responders who took action quickly. We will continue to monitor this situation and hold the Church of Jesus Christ of Latter-Day Saints in the Grand Blanc close.”

ABC News’ Luis Martinez, Chris Looft and Sasha Pezenik contributed to this report.