(CNN) -- Britons, Germans and other tourists on the Spanish island of Mallorca "can feel safe because they aren't targets of the ETA terrorist band," which recently bombed the popular resort, Spain's interior minister said Tuesday.
Police cordon off the route leading to the location of the blasts in Palma de Mallorca.
"People should feel safe because security forces are on top of this," Interior Minister Alfredo Perez Rubalcaba told a nationally-televised news conference in Madrid.
Rubalcaba's comments followed the Sunday explosions on Mallorca of what he said were four small ETA bombs that caused no injuries, and two weeks after an ETA car bomb killed two Civil Guard officers on the same Mediterranean island.
But Rubalcaba said police aren't sure if the ETA militants behind the bombings remain in hiding on the island or have left, and he warned repeatedly that the Basque separatist group could attack again.
"We are on maximum alert in Mallorca and the rest of Spain," Rubalcaba said. "ETA, when it can, attacks, so you can't rule out that they won't attack again. We try to prevent them from doing so." Watch background behind ETA's decades-long struggle »
The island's regional government told CNN on Monday that since the bombings, there have been no "significant" tourist cancellations.
Tens of thousands of Britons and Germans are vacationing on Mallorca, as is Spain's King Juan Carlos and the royal family.
ETA is blamed for more than 800 deaths in its long fight for Basque independence.
But Tuesday, Rubalcaba confirmed that police believe a fourth bomb also was ETA's work. Some local authorities initially thought it might have been just an accidental gas explosion.
Three of the bombs exploded in the restrooms of restaurants and a bar in the main city, Palma de Mallorca, and the other was placed in the restroom of an underground commercial area beneath a main square.
Rubalcaba revealed that at least one of the bombs was in a male lavatory; previously it was reported they were all in women's restrooms.
Rubalcaba said one of the three warning calls on Sunday preceding the attacks came from near Bordeaux, France --- ETA's traditional rear-guard base. The warning calls, he added, provided only scant details about the bombs' locations to police.
The four small bombs were activated by timers, and one of them was hidden in a bar that had been closed since Friday.
Rubalcaba reiterated the government's position that "these attacks don't do anything but strengthen our determination. Those who placed the bombs will spend long years in jail and recent experience says they will."
ETA has been battered over the past 16 months by arrests of four suspected top ETA military chiefs and dozens of militants.
Hours before Sunday's bombs, ETA claimed responsibility for a series of bombings across Spain in June and July: the one that killed two Civil Guard officers, another that killed a police officer, and another that heavily damaged a Civil Guard barracks, slightly wounded dozens, including children.
In a statement released to the Basque newspaper Gara, ETA said those attacks were in retaliation for the Socialist government's crackdown on its ranks.
The group said, "What ETA has been looking for during long decades is a negotiated political solution."
But Rubalcaba recently ruled out a resumption of negotiations that his government tried without success in 2006 during an ETA cease-fire.
Spanish media last week cited numerous politicians and analysts who suggested that ETA's recent spate of violence might be trying to force the government back to the negotiating table.