Minnesota assassin Vance Boelter went to the homes of at least two other state legislators and had a close-call encounter with cops at one of them the same night he shot a pair of lawmakers and their spouses, prosecutors revealed Monday.

Boelter, 57, meticulously planned his attack, researching the victims and their families and conducting surveillance of their homes, Acting US Attorney Joseph Thompson said at a press conference — calling the case “the stuff of nightmares.”

The suspect — who now faces the death penalty if convicted — allegedly first went to the home of state Sen. John Hoffman and his wife Yvette and banged on their front door, shouting, “This is the police! Open the door!” He later told the couple he was actually robbing the home when they realized he wasn’t a real police officer — and then shot them, authorities said.

After wounding the Hoffmans, Boelter drove to the suburb of Maple Grove with the intention of targeting an unidentified state representative, but the lawmaker was on vacation and not home at the time, officials said.

The deranged man next went to the home of state Sen. Ann Rest in New Hope, where he encountered an officer who had been dispatched to the home for a wellness check after learning of the shooting at the Hoffmans, authorities said.

The officer saw what he thought was another squad car parked down the street and tried to engage Boelter in conversation but was unsuccessful, officials said. The suspect was already gone by the time additional cops arrived at the scene to investigate further, Thompson said.

“I have been made aware that the shooting suspect was parked near my home early Saturday morning,” Rest said in a statement Monday. “I am so grateful for the heroic work of the New Hope Police Department and its officers.

“Their quick action saved my life,” she said.

After the close call, Boelter then allegedly traveled to the home of former state House Speaker Melissa Hortman and her husband Mark and parked his vehicle in their driveway. Officers arrived at Hortman’s home to check on her — amid growing concern over the threat to lawmakers — and Boelter got out of his SUV and opened fire, prosecutors said.

He then ran into Hortmans’ house, where he killed the representative, her husband and their golden retriever, Thompson said. After emerging from the home, Boelter engaged in another gun battle with the cops before disappearing on foot.

When police searched his black SUV — which was outfitted with police lights and a police license plate — they found five additional firearms, including assault-style rifles and a “large quantity” of ammunition, officials said.

Pictures taken inside the vehicle shared by authorities show the weapons — which included at least one AK-47 — strewn about the inside of the messy vehicle, which also contained numerous duffle bags and backpacks.

Authorities announced Monday that Boelter was hit with federal murder and stalking charges in addition to the state raps against him — and could face the death penalty because of it.

Thompson gave a rundown of the six federal charges against Boelter: two counts of stalking, two firearms offenses for the non-fatal shootings and two counts of murder for the killings.

The penalties for the six counts range from 20 years in prison to life, with the death penalty on the table for the federal murder charges. Minnesota abolishing the death penalty involving state charges in 1911.