At least four people were killed in a courthouse when the defendant opened fire during a hearing in Italy's Milan city on Thursday, media reported.
A bankruptcy judge, Fernando Ciampi, and a young lawyer, Lorenzo Alberto Claris Appiani, were among those killed, Italian news agency ANSA reported.
The gunman, named as Claudio Giardiello, was arrested at Vimercate, near Monza, by Carabinieri police after he tried to flee on a motorbike, Interior Minister Angelino Alfano said.
Giovanni Canzio, the president of Milan's appeals court, announced that Ciampi was dead.
According to sources, Ciampi was killed inside his office.
The third victim showed no signs of being hit by a firearm and may have suffered a heart attack during the incident, the ANSA report said.
The fourth victim was Giorgio Erba, who died of his injuries after being brought to hospital in a critical condition.
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The gunman, Giardiello, 57, is facing bankruptcy charges along with Erba and Davide Limongelli.
Limongelli was injured in the shooting.
"I was outside a courtroom on third floor and I heard the gunshots. I saw a young man who had been shot to death and later two others lying on the floor," Xinhua news agency quoted a lawyer as telling Rai television channel.
"He used to be a client of mine, but I gave up on him because he was an over-the-top person," Giardiello's former lawyer commented on the shootout.
"Understanding how it was that a man armed with a gun was able to enter the courthouse will be a very serious issue," a columnist of Milan-based Corriere della Sera newspaper, Giuseppe Guastella, told Xinhua.
"There are seven passages in the courthouse, of which three are equipped with metal detectors," Guastella said.
He explained, however, that judges and lawyers are allowed to enter the courthouse without passing through the metal detectors.
Hundreds of courthouse workers, journalists and passers-by gathered outside the courthouse, which is located in a central area of the northern Italian city where the world Expo 2015 will kick off on May 1.