Cesare Battisti, an Italian writer and former leftwing guerrilla fighter extradited to Rome from Bolivia in January, has confessed to murdering four people in the 1970s, after decades of denying involvement.

Prosecutors said Battisti, who until now had always professed his innocence, admitted to killing two people and being the instigator in the murder of two others under questioning at the high-security Oristano prison in Sardinia.

“I am aware of the harm I have done,” Battisti told the prosecutors. “I apologise to the victims’ families, but it was a war.”

Battisti was convicted in Italy in 1979 of belonging to the outlawed Armed Proletarians for Communism and escaped from prison in 1981. He was later convicted in absentia of killing two police officers, taking part in the murder of a butcher and helping to plan the killing of a jeweller.

He had previously admitted to being part of the group but denied responsibility for any deaths.

Battisti spent three decades in Mexico and France, where he was protected by a 1985 law known as the Mitterrand doctrine that offered asylum to about 100 former Italian guerrillas “on the condition that they withdrew from politics”.

In 2004, he skipped bail in France and took refuge in Brazil, where he lived clandestinely for three years until he was arrested in 2007 in Rio de Janeiro. After four years in custody, the then president, Luiz Inácio Lula da Silva, issued a decree refusing Battisti’s extradition to Italy and he was freed.

Jair Bolsonaro, elected Brazilian president in October, promised the far-right Italian interior minister, Matteo Salvini, he would send Battisti back to Italy in order to serve his prison sentence. He said the extradition would reflect to the world his government’s commitment to fighting terrorism.

A supreme court judge in Brazil in December ordered Battisti’s arrest and Salvini celebrated by posting a photo of the former guerrilla on his Facebook page captioned: “the good times are over”.

He added: “My heartfelt thanks to President Jair Messias Bolsonaro and to the new Brazilian government for the changed political climate which, together with a positive international scenario in which Italy has become a protagonist, has enabled this triumph.”

But just when everything seemed ready for his extradition, Battisti once again fled. He was found by special forces a month later in Bolivia.

Waiting for the jet with Battisti on board at Rome’s Fiumicino airport, the Italian authorities erected a stage with a band playing the national anthem, while Salvini and the minister of justice, Alfonso Bonafede, waved for the cameras. Salvini and Bonafede were investigated by the authorities for failing to protect the dignity of the arrested person, but magistrates dismissed the charges.

”The choice to plead guilty finally sheds light on the Armed Proletarians for Communism who acted in one of the most violent periods of our history,” said Milan’s chief prosecutor, Francesco Greco, announcing Battisti’s admission.

Battisti may face life imprisonment, but his lawyers have opposed this possibility, arguing that the former guerrilla fighter had been extradited under the agreement with Brazil, where life imprisonment is unconstitutional.