"At about 2:25 we had a disturbance on the third floor of the courthouse. A person has shot two bailiffs, they are both deceased, and a deputy sheriff who is at the hospital right now being treated in the emergency room," Bailey said in a brief news conference.
"The suspect has been shot and killed," he added. He said the death of his friends was "terrible."
The suspect was an in-custody inmate who took a gun off a law enforcement officer and began shooting, ABC affiliate WZZM reported. Bailey said it was not clear how the suspect got the weapon.
The shootings of the bailiffs and the deputy came four days after four Dallas police officers and one Dallas transit police officer were killed by a sniper who claimed to be motivated by police use of lethal force against African-Americans.
St. Joseph is a town on the shores of Lake Michigan, across the lake from Chicago.
The wounded sheriff's deputy and several civilians also injured in the shooting are being treated at Lakeland Regional Medical Center, spokeswoman Jessica Hines said by telephone. She declined to detail their condition.
Buildings in the area were put on lockdown after the shooting, according to media reports.
"MSP (Michigan State Police) has secured the scene at the Berrien County courthouse and started its investigation into the shooting that occurred this afternoon," Governor Rick Snyder said in a Tweet.
Public information officers at Michigan State Police and the Berrien County Sheriff's Department declined to give any additional details.
Chris Gautz, public information officer for the Michigan Department of Corrections, tweeted that all Department of Corrections staff are safe and accounted for.
A witness, Gretta Volkenstein, told CNN that buildings and government agencies in the area were all on lockdown.