A man who is accused of killing a Democratic state lawmaker and her husband, and injuring another state lawmaker and his wife, allegedly attempted to kill two other state lawmakers, Minnesota officials said on Monday.

Vance Luther Boelter, 57, who is charged with fatally shooting Melissa Hortman, a Minnesota Democrat, and her husband, Mark, visited the homes of three other state lawmakers in "truly chilling" attacks, US Attorney Joseph H Thompson said.

Mr Boelter, who police said researched the victims and their families beforehand, allegedly had planned for a larger scale attack, which police thwarted.

He appeared in court on Monday afternoon to face six federal charges, and possibly the death penalty, if he is found guilty.

Mr Boelter was wearing an orange jumpsuit when he arrived in court in St. Paul on Monday afternoon. He said he cannot afford a lawyer and will have a federal defence lawyer.