As the investigations continued Tuesday into two separate mass shootings allegedly perpetrated over the weekend by Marine veterans wielding assault rifles, one in Michigan and the other in North Carolina, numerous questions remain unanswered -- chief among them motive.

The attacks at a waterfront bar in Southport, North Carolina, and at a Church of Jesus Christ of Latter-day Saints in Grand Blanc, Michigan, left seven people dead and more than a dozen injured.

So far, federal and state investigators have shed little light on why the suspects, who had both been deployed to the Iraq War, allegedly committed the unrelated attacks.

No additional news conferences have been scheduled for Tuesday in either case.

Investigators in Michigan alleged that around 10:25 a.m. on Sunday, 40-year-old Thomas Jacob Sanford of Burton, Michigan, rammed his pickup truck through the doors of the LDS church in Grand Blanc, Michigan, and let loose a barrage of gunfire on hundreds of people worshipping there before setting fire to the chapel.

Four people were killed in the attack and eight others were injured, two critically, officials said.

Sanford, who police said acted alone, was killed during a gunfight with police, authorities said.

Reuben Coleman, acting special agent in charge for the FBI Detroit field office, said at a news conference on Monday that the attack is being investigated as "an act of targeted violence."

"We are continuing to work to determine a motive," said Coleman.

Asked by reporters why the attack isn't considered an act of domestic terrorism, Coleman said, "Once we're able to ascertain definitively what we have, that's when we'll make those statements. But right now, this is an act of violence."

By comparison, federal officials were quick to call a Sept. 24 sniper attack on a U.S. Immigration and Customs Enforcement facility in Dallas that left two detainees dead, an act of terrorism.

Expounding on what he meant by "targeted violence," Coleman said, "I mean targeted within our state and within our community."

Coleman said investigators interviewed more than 100 victims and witnesses in the first 24 hours of the investigation, but he shared little on what has been learned about the motive.

In an interview with ABC News, Kris Johns, a candidate for Burton City Council, said he met Sanford while campaigning just days ago and that Sanford expressed anger towards Mormons.

Twin brothers Peter and Francis Tersigni of Michigan told ABC News on Tuesday that they grew up in Michigan with Sanford, who they referred to as "Jake," and confirmed he harbored an animus against Mormons.

The brother said that after leaving the military in 2008, after serving four years in the U.S Marines, Sanford relocated to Utah for a job and dated a woman there who was Mormon, and that Sanford had talked about converting to the religion.

Peter Tersigni, who said he was once best friends with Sanford, said that while living in Utah, Sanford got heavily into drugs, specifically methamphetamine.

"It messed his life up and it messed his head up. And it just happened to be at the time he was around Mormons," Peter Tersigni said.

Peter Tersigni said Sanford moved back to Michigan around 2010. He said Sanford would go off on tangents about Mormons, usually while drinking alcohol, disparaging the church, but never attributing his negative feelings about the religion to his breakup with the Mormon woman he had dated.

The North Carolina attack occurred around 9:30 p.m. on Saturday, about 12 hours before the Michigan mass shooting, and investigators alleged it was carried out by 40-year-old Nigel Edge, whom they described as a "lone wolf," who opened fire from a boat on an outdoor waterfront bar in Southport, killing three people and injuring five others.

"We believe this was a targeted location," Southport Police Chief Todd Coring said during a news conference on Sunday. "This is highly premeditated from what we're seeing at this time."

But Coring and other law enforcement officials have yet to release more details on the possible motive.

Edge was detained shortly after the shooting in nearby Oak Island as he was loading his boat at a public boat ramp, officials said.

Edge is charged with three counts of first-degree murder, five counts of attempted murder; and five counts of assault with a deadly weapon with the intent to kill or injure, officials said. He has yet to enter a plea to the charges and was appointed a public defender during his first court appearance on Monday.

Prosecutors said they are weighing whether to pursue the death penalty against Edge.

Edge was awarded the Purple Heart after being wounded in combat, according to the U.S. Marine Corps.

The suspect legally changed his name in 2023 from Sean Debevoise, writing in a petition to the court, "There have been a lot of events in my life that I don't understand. Therefore, I do not trust my family, and I would feel more comfortable starting my life in a new path with a new name."

Investigators said in a criminal complaint that Edge opened fire on the Southport bar with a short-barrel AR rifle that was equipped with a suppressor, a scope and a folding stock. Authorities have yet to disclose how many shots were fired in the incident.

In the Michigan attack, officials have only described the weapon allegedly used by Sanford as an "assault rifle."

Authorities haven't said publicly how either Edge or Sanford obtained their weapons. Authorities have also not commented on how long they believe each suspect planned their respective attacks.

After he was arrested, Edge told investigators that he was injured in the line of duty as a Marine and said that he suffers from post-traumatic stress disorder, according to Coring.

Jon David, the local district attorney, said at a news conference on Sunday, that Edge had some minor contacts with law enforcement, "but nothing significant in his past which would give us any indication that we was capable of such horror."

During a news conference on Monday, officials in Michigan said Sanford had prior arrests for burglary and operating a vehicle while intoxicated.

Authorities have not disclosed if either Edge or Sanford had any previous connections to the targets they allegedly attacked.

In the Michigan shooting, witnesses said they did not recognize Sanford as someone associated with the chapel.

In the Southport shooting, authorities have not said why Edge allegedly opened fire on the American Fish Company bar or if he was a patron of the establishment. But Coring asked that anyone who was at the bar on Friday, the day before the shooting, or Saturday night to contact investigators immediately.

Regarding the Michigan attack, investigators recovered four improvised explosive devices from the pickup truck Sanford allegedly used to plow into the LDS chapel, James Deir, the special agent in charge of the Detroit field office of the federal Bureau of Alcohol, Tobacco, Firearms and Explosives, said on Monday.

ABC News has learned from sources familiar with the investigation that the IEDs were made from consumer-grade fireworks.

Investigators believe the suspect allegedly set the chapel ablaze after dousing it with gasoline, but it remains unclear what the suspect planned to do with the IEDs.

Investigators have also not commented on the possible meaning of the American flags the Michigan suspect had attached to the rear bed of the pickup truck used in the attack.

Deir said the ATF's National Response team, comprised of a cadre of world-renowned arson and explosive investigators are involved in the investigation.

ABC News' Andy Fies and Alex Perez contributed to this report.