Boelter is accused of posing as a police officer and fatally shooting former Democratic House Speaker Melissa Hortman and her husband, Mark, in their home early Saturday in the northern Minneapolis suburbs.
Authorities say he also shot state Sen. John Hoffman, also a Democrat, and his wife, Yvette. They were injured at their residence 9 miles away.
"One man's unthinkable actions have altered the state of Minnesota," Democratic Gov. Tim Walz said at a news conference after Boelter's arrest.
The manhunt began when Brooklyn Park officers went to check on Hortman's home and saw her husband gunned down before the shooter fled.