Minnesota police say two officers and a paramedic have been killed in a shooting at a family home.
The gunman, who was barricaded inside his Burnsville home with seven children, opened fire as officers tried to negotiate with him.
The dead officers were named as Paul Elmstrand and Matthew Ruge, both 27, and the paramedic was Adam Finseth, 40.
The attacker also died in the incident, which followed a domestic abuse call in the early hours of Sunday morning.
Burnsville police chief Tanya Schwartz said her whole force was "hurting", while Minnesota Governor Tim Walz described it as a "tragic loss for our state".
State authorities said officers were called at about 01:50 local time (07:50 GMT) to the address in Burnsville, a city about 15 miles (24 km) south of central Minneapolis.
They were responding to a "report of a domestic situation" involving an armed man, said Supt Drew Evans from the Minnesota Bureau of Criminal Apprehension.
The responders later learned seven children, aged two to 15, were also "barricaded" in the property. They went on to spend "quite a bit of time negotiating with this individual".
The attacker then opened fire, killing the three victims and inflicting non-life-threatening injuries on a fourth policeman, who was named as Adam Medlicott.
Details of exactly what happened next are not yet clear, though Supt Evans confirmed that the police "did return fire". Local media said the gunman killed himself.
He is said by Supt Evans to have had "several guns and large amounts of ammunition", and shot at the first responders from different parts of the home.
He has not yet been named. He was found dead at about 08:00, which left other inhabitants of the home able to escape unharmed.
Neighbours on the quiet residential street described their fear as the incident escalated. Jason Skog recalled seeing a large police and Swat presence in his neighbourhood.
Describing the sound of explosions and gunfire, he said it quickly became clear that "something bad was taking place".
An investigation is under way into what happened, while tributes have been paid by emergency workers in the local area and beyond. A candlelit vigil was held for the victims on Sunday night.
The incident came just days after another mass shooting in another Midwestern state, during which one person was killed and 21 injured in Missouri.
The victims were among a crowd who had been watching a victory parade by the Kansas City Chiefs after the NFL team won the Super Bowl earlier this month.
A pair of teenagers have been charged over the attack.
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