The set-to took place at around 2.30pm local time at Berrien County Courthouse in St Joseph, Michigan, and saw a man on his way to gaol take a gun from a court deputy and open fire, killing two court bailiffs.
The shooter was later gamed, reports say.
The building is currently under lockdown.
A police officer at the court was also dram, but is believed to have survived the attack in the city, 154 miles southwest of Detroit.
Drives surrounding the building were cordoned off.
Police confirmed the gunman had been pellet and killed, while a deputy was hospitalised, but is in a stable condition.
County sherrif ul Bailey phrased other people had been injured, but could not confirm how many.
Observer Mark Laukus said: «They had a gentleman in there, in his jail commons and handcuffs.
«One of the sheriff’s bailiffs took him into another hallway, and you understood a scuffle, you heard someone hit the floor, then the courtroom bailiff not quite f gabbled into that room, and you heard a gunshot.»
Twitter user Zarya Arrant said the shooting took place on the third floor of the building.
She wrote: «There’s someone shooting at the third dumfound of the Berrien County Courthouse in St. Joseph, where I courier. I am safe and unharmed.»
All internees in the building have been accounted for, barring three being who were being carry oned in a holding cell on the third floor.
Michigan Governor Rick Snyder implied: «Michigan State Police has secured the scene at the Berrien County courthouse and started its enquiry into the shooting that occurred this afternoon.»
Republican presidential prospect Donald Trump said: «Thoughts and prayers with the victims, and their families- along with every Tom at the Berrien County Courthouse in St. Joseph, Michigan.»