The files will be opened up to “contribute to the reconstruction of events in recent decades that have marked the country’s history,” Prime Minister Matteo Renzi wrote in a letter published online.
The premier said the documents, “relating to the worst events over a period of 30 years,” will be deposited in the state archives.
They includes files on the “Years of Lead” in Italy, marked by terrorist bombings in the 1970s and 1980s across the country.
In one of the worst attacks of the period, 85 people were killed and over 200 wounded when a bomb exploded at Bologna train station. The Bologna Massacre, as it has become known, was attributed to right-wing extremists.
Italians will also be able to access secret documents relating to the 1984 mafia bombing of the Rapido 904 train in central Italy. The attack killed 16 people and injured more than 200 in central Italy, just two days before Christmas.
A number of other national tragedies are due to come under the spotlight, including two bomb attacks in central Milan.
Declassifying the documents will also shed light on a mysterious 1980 plane crash. Itavia Flight 870 crashed close to the island of Ustica while en route to Sicily, killing all 81 passengers. The cause of the accident has never been determined, leading to a wave of conspiracy theories.