Madrid, Spain (CNN) -- Police arrested four suspected members of the Basque separatist group ETA early Tuesday, seizing 200 kilograms (440 pounds) of explosives, Spanish Interior Minister Alfredo Perez Rubalcaba said.
The pre-dawn raids took place in Bilbao, the largest Basque city in northern Spain, and the nearby town of Galdakao, where police also found bomb-making components, Rubalcaba said in a nationally televised news conference.
Spanish media, citing police sources, said the suspects are linked to ETA's killing two years ago of a police intelligence officer in the Basque region, but Rubalcaba only would confirm that investigators were analyzing possible links to a series of unsolved ETA attacks.
The arrests come during a unilateral ETA cease-fire which the outlawed group declared January 10, promising it would be "permanent, general and verifiable."
But the Socialist government has repeatedly said it would not ease the police pressure against ETA, which in 2006 broke an earlier "permanent" cease-fire with a car bomb at Madrid's airport that killed two men and caused extensive damage.
Rubalcaba said the arrests Tuesday have "enormous importance" because the four are suspected of operating clandestinely for ETA, under the cover of living and working "as normal citizens, integrated into Basque society, which makes it very difficult for police to find them."
Police have worked to locate these suspected clandestine operatives for more than two years, since the arrest of a suspected ETA military chief in November 2008, Rubalcaba said.
The suspects include two men and two women, ranging in age from 31 to 37 years old, the Interior Ministry said in a statement, adding that police also seized three firearms and forged identity documents for the Basque regional police force in the raids.
ETA is blamed for more than 800 deaths in its decades-long fight for Basque independence and is listed as a terrorist group by the United States and the European Union.
ETA's last killing in the Basque region was in 2009. A senior police intelligence officer, Eduardo Puelles, died when a limpet bomb -- which attaches to its target magnetically -- exploded under his car. It is among a number unsolved ETA attacks over the past few years.
On January 18, a week after ETA's latest cease-fire announcement, police arrested 10 people, most of them for alleged links to a shadowy group called Ekin, which authorities say passes directives from ETA's leadership to a string of equally shadowy support groups.
"The government's anti-terrorist policy has not changed at all because ETA has not definitively abandoned its weapons. And as long as it doesn't, the police will continue to arrest them, one after another," Rubalcaba said.