Prosecutor Speaks Out on Possible Motive in Minnesota Shootings That Killed, Wounded Lawmakers

Investigators are still working to confirm the motive behind Saturday morning’s shootings in Minnesota, which killed one state lawmaker and her husband and wounded another lawmaker and his wife.

Vance Boelter was arrested Sunday night and charged in connection with the shooting deaths of Democratic Minnesota lawmaker Melissa Hortman and her husband Mark, officials with the Hennepin County Attorney’s Office announced Monday. The shooter, 57, is also accused of shooting Democratic state Sen. John Hoffman and his wife Yvette.

At a federal prosecutor’s news conference on Monday, U.S. Attorney Joseph Thompson told reporters that “it’s pretty clear from evidence that he’s been planning these attacks for quite some time,” and that “voluminous writings” had been “found both in his car and his house, about his planning, lists of names and individuals.”

“But I have not seen anything involving some sort of political screed or manifesto that would clearly identify what motivated him,” Thompson added. “Obviously, his primary motive was to go out and murder people. They were all elected officials. They were all Democrats. Beyond that, I think it’s just way too speculative for anyone that’s reviewed these materials to know and to say what was motivating him in terms of ideology or specific issues.”

Thompson announced that on the federal level, Boelter has been charged with two counts of murder, two counts of stalking and two firearm offenses — one in relation to the shooting that killed Melissa and Mark Hortman, and another in the shooting that wounded John and Yvette Hoffman — according to CNN and NBC News.

CNN reports that the two federal murder charges against Boelter carry the potential for death penalty sentences, though Thompson said it’s “too early to tell” whether federal prosecutors will seek the death penalty.

At another news conference on Monday, Hennepin County Attorney Mary Moriarty said Boelter has also been charged with second-degree murder and attempted murder charges at the state level. “These are the highest charges we can file by complaint and it needed to be done quickly in order to get the warrant out,” Moriarty explained. “Our office intends to pursue first-degree murder charges against Mr. Boelter.”

Want to keep up with the latest crime coverage? Sign up for PEOPLE’s free True Crime newsletter for breaking crime news, ongoing trial coverage and details of intriguing unsolved cases.

Boelter allegedly impersonated a law enforcement officer during the shootings, the officials said. Thompson shared a photo of Boelter allegedly taken from the Hoffman family’s home security camera at their front door. Boelter allegedly arrived in a black SUV, wore a tactical vest, and carried a flashlight and a gun. He also wore a “hyper-realistic silicone mask,” according to Thompson, who called the footage “chilling.”

“Boelter planned his attack carefully,” Thompson said. “He researched his victims and their families. He used the Internet and other tools to find their addresses and names, the names of the family members. He conducted surveillance of their homes and took notes about the location of their homes.”

Thompson added: “It’s no exaggeration to say this is the stuff of nightmares.”