Vance Boelter, 57, faces state and federal murder charges after his arrest on Sunday night following a massive two-day manhunt that was the largest in state history.
He is accused of fatally shooting Melissa Hortman, the top Democrat in the Minnesota House, and her husband Mark, in their home on Saturday.
Boelter is also charged with shooting and wounding Democratic state Senator John Hoffman and his wife Yvette in their home a few kilometres away.
Prosecutors said on Monday that Boelter visited the homes of two other politicians disguised as a police officer, hunting more targets.
Investigators said they recovered notebooks from his car and residence that included the names of dozens of Democratic legislators, along with abortion rights advocates.
They also revealed that he sent a text to his family hours after the shooting that said in part, "Dad went to war last night".
"Political assassinations are rare," Joseph Thompson, Minnesota's acting US attorney, said at a news conference.
"They strike at the very core of our democracy."
Thompson said a precise motive was not yet clear.
Contrary to earlier news reports, authorities have not discovered a "Unabomber-style manifesto," he said.
Boelter made a brief appearance in St Paul federal court on Monday to face murder and other charges, which could result in the death penalty.
Wearing an orange jumpsuit, he told a judge he understood the charges against him and remained composed throughout the 15-minute hearing.
He did not enter a plea and will return to court on June 27.
Boelter's court-appointed lawyer Matthew Deates declined to comment after the hearing.
Boelter also faces separate state murder charges in Hennepin County.
"Vance Luther Boelter embarked on a planned campaign of stalking and violence, designed to inflict fear, injure and kill members of the Minnesota state legislature and their families," an FBI affidavit unsealed on Monday read.
The attacks began about 2am on Saturday when a gunman wearing a police-style tactical vest and a "hyper-realistic silicone mask" knocked on the Hoffmans' door in Champlin, announced himself as a police officer and then shot the couple multiple times, according to prosecutors.
He was driving an SUV outfitted with police-style lights and a fake licence plate that read "POLICE".
Boelter then travelled to the home of another state lawmaker in Maple Grove, where he rang the doorbell at 2.24am, Thompson said.
The official, whom prosecutors did not name, was not home.
Boelter also visited the home of an unnamed state senator in New Hope, prosecutors said.
An officer dispatched to the house to check on the senator after the Hoffman shooting saw Boelter parked outside and believed he was another police officer.
When Boelter ignored the officer's greeting, the officer went to the door to wait for additional police, Thompson said.
Boelter had left by the time they arrived.
Shortly after, police went to the Hortmans' house in Brooklyn Park as a precaution.
The arriving officers saw the suspect shoot Mark Hortman through an open door about 3.35am and exchanged fire with him before he fled out the back, according to prosecutors.
Melissa Hortman was found dead inside.
When police searched Boelter's SUV, they discovered three assault-style rifles, a handgun, a gold police-style badge and a target list, according to authorities.
Boelter's notebooks showed he had planned the attacks for some time.
He had the names and, in some cases, home addresses for more than 45 elected officials, "mostly or all Democrats," according to the affidavit.
In one book, Boelter noted the Hortmans had two children and added surveillance details about their house, writing, "Big house off golf course 2 ways in to watch from one spot," the affidavit said.
