Minneapolis, Minnesota - Police and FBI agents waged a huge manhunt Sunday for a gunman who killed a Democratic state lawmaker and her husband in Minnesota in what officials called a politically motivated assassination.

As the search stretched into its second day, police appeared to close in on the shooter, finding a car described as related to him, but not the man himself.

Authorities are looking for Vance Boelter, who is suspected of killing state Representative Melissa Hortman and her husband, as well as wounding state Senator John Hoffman – also a Democrat – and his spouse in attacks on Saturday.

Officials said Boelter impersonated a police officer as he came to the homes of the couples near Minneapolis and shot them. A manifesto and a list of other Democratic politicians and potential targets was also discovered, with a close friend confirming to CNN that the 57-year-old was a conservative Trump supporter.

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Boelter fled on foot after exchanging gunfire with officers after the second shooting.

On Sunday officers located another car related to Boelter in a rural area about a 90-minute drive west of Minneapolis, the Sibley County Sheriff's Office told AFP.

Residents were warned of the find and agents are scouring the area, the office said without explaining how the vehicle is related to the suspect.

Officials have issued security alerts in South Dakota and other states as the hunt proceeds, US Senator Amy Klobuchar of Minnesota said Sunday as she mourned her slain friend, Democratic state representative Melissa Hortman.

"I am concerned about all our political leaders, political organizations," she said.

"It was politically motivated, and there clearly was some throughline with abortion because of the groups that were on the list, and other things that I've heard were in this manifesto. So that was one of his motivations."

On Saturday the FBI released a photo that appears to show Boelter wearing a latex mask as he stands outside the home of one of the lawmakers.

It is offering a $50,000 reward for information leading to his arrest and conviction.

The shootings came on the day millions of protesters across the country took to the streets to rally against Trump as the president presided over a big military parade in Washington for his birthday.

Trump has condemned the killing of Hortman and her husband Mark and the wounding of state Senator John Hoffman and his wife Yvette.

In a conversation Sunday with ABC News, Trump was asked if he planned to call Minnesota Governor Tim Walz, who was Kamala Harris's running mate in the election Trump won last year.

"Well, it's a terrible thing. I think he's a terrible governor. I think he's a grossly incompetent person," the president said. "But I may, I may call him, I may call other people too."

Since returning to office, Trump has whipped up hatred against immigrants, minorities, political opponents of all stripes, and institutions standing in the way of his far-right agenda.

He has backed up his words with actions, unleashing ICE agents to detain and deport people without due process, while deploying troops on the streets of Los Angeles to suppress an uprising against his authoritarian regime.