(CNN) -- Police in Portugal this week seized more than 3,000 pounds of explosives from a house used by the Basque separatist group ETA, the Spanish Interior Ministry said Saturday.
Additionally, other suspicious material was found in boxes inside the home in Obidos, about 60 miles north of Lisbon, said Lt. Gen. Nelson Dos Santos, commander of the Portuguese National Republican Guard on Friday.
"The items that we have found and that are now being analyzed by the national anti-terrorism unit ... lead us to believe that we are dealing with terrorists," said Jose Almeida Rodrigues, head of Portugal's judicial police.
The discovery was made Thursday and came three days after guards attempted to stop an SUV whose occupants led them on a chase.
The SUV was later found abandoned and authorities determined it had been stolen a year earlier. Several Portuguese license plates and four explosives detonators commonly used by the ETA were found inside, the Interior Ministry said in a statement.
A receipt from a purchase made January 17 led security forces to analyze security cameras at the establishment where the purchase was made and identified the individual who made the purchase as Andoni Cengotita Bengoa Fernandez, a suspected ETA member, the ministry statement said.
ETA is blamed for more than 800 deaths in its long fight for Basque independence. It is listed as a terrorist group by Spain, the European Union and the United States.
Spain's Interior Minister Alfredo Perez Rubalcaba warned in December that ETA could be planning a major attack or high-profile kidnapping, and the government raised its terror alert level.
Earlier this month, four of the most-wanted suspected ETA members were arrested in Portugal and France, according to the Interior Ministry.
ETA, founded in 1959, is an acronym for Euskadi ta Askatasuna, which means "Basque Homeland and Liberty" in the Basque language.