MADRID, Spain (CNN) -- Three bombs blamed by police on Basque separatist group ETA exploded early Thursday in northern Spain's Basque region, causing damage but no reports of casualties.
The first -- and largest -- explosion occurred without warning around midnight (6 pm ET) in an industrial pavilion for vehicles from Spain's Ministry of Labor, a Basque regional police spokesman told CNN.
That blast happened in the Basque town of Arrigorriaga and caused extensive damage to the site, which is in an industrial area.
The other two bombs exploded half an hour later, at 12:30 a.m. local time, near a Basque regional government labor institute in the large Basque city of San Sebastian.
Those two bombs were preceded by warning calls in the name of the ETA, giving police time to cordon off the area. They caused little damage, according to the spokesman.
The latest bombs follow similar recent attacks also blamed on ETA.
On April 20, a bomb preceded by an ETA warning call damaged a recreation center of the ruling Socialist Party in the Basque town of Elgoibar. It caused no injuries.
Days earlier, seven Basque police officers were slightly hurt when a bomb exploded on the outskirts to the largest Basque city, Bilbao.
ETA -- blamed for more than 800 deaths in its long fight for Basque independence -- threatened the Socialist Party after the re-election in March of Prime Minister Jose Luis Rodriguez Zapatero.
On March 7, two days before Zapatero won re-election, ETA was blamed for the fatal shooting of a former town councilman, Isaias Carrasco, from the Socialist Party in the Basque town of Mondragon.
The latest trio of overnight bombings - as Spain observes the May 1 international labor day holiday - came just hours after a Spanish judge on Wednesday jailed the mayor of Mondragon, Inocencia Galparsoro on charges of collaborating with ETA.
Galparsoro has refused to condemn the killing of the ex-councilman Carrasco, despite the insistence of the Socialists and other parties. She recently avoided losing her job, when a censure motion in the town hall failed to win enough votes to oust her.
Galparsoro is widely seen to be associated with the leftist Basque Nationalist Action party, ANV, but she denied that she's a member of ANV in testimony before Judge Baltasar Garzon, according to a copy of his court order, viewed by CNN, that sent her to jail.
ETA declared a unilateral cease-fire in March 2006, raising hopes for an end to the violence. However, an ETA bomb at Madrid's airport in December 2006 killed two men, destroyed a parking garage -- and ended the peace process.
ETA officially terminated the cease-fire in June 2007, and since then the government has blamed it for three killings: two undercover Spanish Civil Guards who were shot dead in France while on an anti-terrorist operation against ETA, and the former councilman killed on March 7. E-mail to a friend