An envelope stuffed with gunpowder exploded today at a post office in eastern Spain, injuring four postal workers, officials said.
A government spokesman said authorities believed a radical group was responsible and did not rule out the Basque separatist group ETA.
Authorities are on alert for a possible ETA attack timed to coincide with Sunday's municipal elections. A Basque separatist party has been stricken from the ballot on the grounds that it formed part of ETA, Western Europe's most active guerrilla group.