The most recent blasts came ahead of the 50th anniversary of Eta's founding
The Basque separatist group, Eta, has claimed responsibility for a series of recent bombings in Spain, including one which killed two Civil Guards in July.
In a statement sent to the Basque newspaper, Gara, Eta said it was also behind a car bomb attack which killed a police inspector two months ago.
It said the policeman, Eduardo Puelles Garcia, was a "despicable torturer".
Eta has been blamed for more than 820 deaths during its campaign for an independent Basque homeland in Spain.
The most recent bombings, on the island of Majorca and the northern town of Burgos, came ahead of the 50th anniversary of its founding.
The two Civil Guards who were killed in the Majorcan resort town of Palmanova on 30 July, Diego Salva and Carlos Saenz de Tejada, had been inside a patrol car parked outside their barracks when a bomb planted underneath it exploded.
Several people were also injured by the powerful blast on the busy road, which sent the vehicle flying through the air and set it on fire.
The attack was the deadliest since two Spanish undercover policemen were shot during an operation in France in December 2007.
Although Eta did not claim responsibility for the Majorca blast at the time, Spain's government said it bore the hallmarks of the separatists.
Experts say a similar method was used in Eta's fatal bombing in June, when Mr Puelles Garcia was killed by a car bomb in Bilbao.
Eta said Mr Puelles Garcia had been "the chief of police operations against numerous leftist independence activists and against pro-independence youths for the past decade, and the co-ordinator of different operations against Eta".
"Sometimes, as the most experienced and despicable torturer in the commissariat... he also pressured independence activists into collaborating by threats," it said.
Eta also claimed responsibility for the car bomb which exploded outside a police barracks in Burgos on 29 July, injuring 40 people, and an attack on the offices of the governing Socialist Party in the Basque town of Durango on 10 July.
But the group insisted that it was not seeking "to impose any project, unlike successive Spanish leaders".
"It is Eta, which has over the long decades sought dialogue and political solutions," the statement added.
The Socialist Party-led Spanish government declared the peace process "finished" after a bomb planted by Eta killed two Ecuadorean men at a Madrid airport car park in December 2006.