Japanese prosecutors on Monday decided not to charge former prime minister Naoto Kan and TEPCO bosses over the initial stages of the nuclear disaster at Fukushima.
The decision still leaves open the possibility that they will face criminal charges over other aspects of the disaster, including recent leaks of contaminated water from the crippled power plant.
Local residents and activists had filed a criminal complaint against Kan and his ministers alleging professional negligence over the accident.
They had also requested prosecutors charge former top executives of Tokyo Electric Power (TEPCO), the operator of the plant, and Haruki Madarame, former chief of the Nuclear Safety Commission.
The complaint alleged that the government officials and TEPCO executives failed to take necessary measures to shield the plant against the March 2011 tsunami.
It also held them responsible for a delay in announcing data predicting how radiation would spread from the facility in the aftermath of the accident.
But prosecutors decided to exempt all of them, saying in a statement that TEPCO could not predict an earthquake and tsunami of that size, while government officials were not legally responsible over their post-quake response.
Campaigners can appeal against the decision at court, which has the power to order the defendants be tried. Activists have said they intend to follow this route.
The huge tsunami, which was triggered by a 9.0-magnitude earthquake, crashed into Fukushima and swamped cooling systems, sparking meltdowns that spewed radiation over a wide area.
No one is officially recorded as having died as a direct result of the radiation released by the meltdowns, but some Fukushima residents committed suicide citing concerns over radiation, while others died during evacuation.
Tens of thousands of people are still unable to return to their homes around the plant, with scientists warning some areas may have to be abandoned.
A parliamentary report has said Fukushima was a man-made disaster caused by Japan's culture of "reflexive obedience" and not just by the tsunami that hit the plant.
The beleaguered plant operator is still struggling to stop polluted water leaking into the ocean, with up to 300 tonnes of mildly radioactive groundwater making its way into the sea every day.