The gunman, Claudio Giardiello, 57, was arrested near Monza after trying to flee on a motorbike, Interior Minister Angelino Alfano said. Prosecutors said he planned to kill others related to a failed property venture. [Photo/IC]

"I wanted to take revenge on those who ruined me," Giardiello, 57, was quoted as telling police who questioned him after the arrest. The man is reportedly separated from his wife with whom he has a daughter and a son.

"I was in the building for work and I was going upstairs when I suddenly saw a number of people panicking, so I started running away too," an eyewitness, Dimitri Bezsmertnyy, told Xinhua.

"At the beginning I did not realize what was happening, then I saw hundreds of people assembled in front of the courthouse and I learnt about the shooting," he said.

But how was it possible for an armed man to enter the courthouse? Giuseppe Guastella, a columnist of Milan-based Corriere della Sera newspaper, told Xinhua there are seven passages in the courthouse, of which three are equipped with metal detectors, while judges and lawyers are allowed to enter just showing their pass.

According to early investigations, Giardiello was able to enter the courthouse showing a fake pass thus avoiding checks.

"The control system cannot afford to have holes and flaws like the ones at the Milan courthouse. We must ascertain who, how and why did wrong. Something did not work," Prime Minister Matteo Renzi said.

An investigation has was opened to find out more about what Justice Minister Andrea Orlando defined as "an amalgam of serious mistakes" that occurred in the heart of the city that will host the next world exposition from May 1 to Oct 31.