Friedrich Heinz "Fritz" Leibacher was born on July 21, 1944, in Zug, Switzerland, into a middle-class family. He had two brothers and a troubled childhood. At the age of 13, he was arrested for publicly firing a rifle and threatening his mother. His parents sent him to a reformatory, and by 16, psychiatrists labeled him as a potentially dangerous psychopath.

Leibacher struggled to hold steady jobs. He worked as a clerical assistant in Zurich but had a long criminal history from 1957 to 1969, which included theft, harassment, trespassing, child molestation, and forgery. Despite his criminal past, his record was eventually erased, which allowed him to legally buy firearms.

In 1965, he briefly studied law at the University of Zurich but was arrested in Turkey for smuggling watches and spent seven months in prison. His personal life was also unstable. He married three Dominican women, and each marriage ended because of his abusive behavior. He had three children from these relationships.

In 1995, he started receiving an invalidity pension for medical conditions, including tinnitus. In 1996, doctors diagnosed him with antisocial personality disorder, possible schizophrenia, and probable alcoholism.

On October 17, 1998, Leibacher threatened a bus driver with a gun during an argument. This led to legal action against him, which deepened his hatred for local authorities. Over the next few years, he sent letters and complaints, accusing officials of corruption and demanding compensation. His messages became more aggressive, and he even reached out to Amnesty International, claiming he was being mistreated.

In 2001, Leibacher made final preparations for what he called his "Day of Reckoning." He closed his bank accounts, sold his home and stocks, and made funeral arrangements, asking that his ashes be scattered over the Atlantic. The day before the massacre, he left a key to a locker containing his case files with his lawyer, wrote a will in the Dominican Republic, and sent a farewell letter to his mother.

Despite being under police surveillance, Leibacher legally purchased multiple firearms, including a pump-action shotgun just nine days before the attack.

On September 27, 2001, at 10:30 a.m., he entered the Zug cantonal parliament building dressed as a police officer. He carried a SIG SG 550 assault rifle, a Remington Model 870 shotgun, a Smith & Wesson revolver, a SIG Sauer pistol, and a homemade bomb. As the government officials were meeting inside, he stormed the assembly hall and shouted, "Attention! This is a police operation!" before opening fire.

Leibacher fired 91 shots, killing 14 people, including three members of the Executive Council and eleven legislators. He also injured 18 others. After causing massive destruction, he detonated a homemade bomb before turning his gun on himself. The massacre lasted just four minutes, ending at 10:34 a.m.

Leibacher left behind a suicide note titled "Day of Reckoning for the Zug Mafia," where he accused local authorities of corruption and portrayed himself as their victim. His attack deeply shocked Switzerland, leading to national mourning and calls for stricter security and gun laws.