ASUNCION, Paraguay – A leftist guerrilla group has claimed responsibility for a bomb that injured five people as Paraguay's third bombing in a week raised alarm about increasing activity by the self-styled Paraguayan People's Army.

A handwritten letter left nearby vowed to continue anti-government attacks and to show no mercy for police shootings of their comrades.

Interior Minister Rafael Filizzola flew to the scene Monday hours after the bomb exploded just before midnight. He vowed no retreat in the effort to jail the guerrillas and dismantle their organization, known by the Spanish initials EPP.

The latest homemade bomb was left in a backpack outside a police station in Horqueta, a small town in northern Paraguay that is home to fugitive members of the EPP. Someone detonated it by remote control as four officers sat in a police vehicle nearby. All four were expected to recover, although one had serious eye damage, special forces commander Elizardo Rojas said. A fifth victim — a motorcyclist passing by — sought treatment for hearing damage at a hospital and was being sought as a witness.

Filizzola said "the bomb was highly powerful, because private homes, neighbors of the police station, suffered material damage."

"For the government, this was a criminal attack with terrorist characteristics," he said, describing the group as "a band of delinquents." he added.

A handwritten note left nearby said the bombing was meant to avenge the deaths of two EPP members who were killed in police firefights last year. "Our dead are more alive than ever," it said, warning: "We won't forget nor forgive. Forget the word mercy. We will apply a high cost for the festival of tortures and the murder of our comrades."

Filizzola led an intense campaign last year to crack down on the EPP, whose ideology about providing for the very poor has won them sympathy and even outright support in impoverished northern Paraguay. Some high-profile arrests were made, but at least three main leaders remain at large.

The government has purchased or received upgraded police equipment from Spain and the United States, and sent 250 special police forces to hunt down EPP members. But the campaign wound down by October, with most of the special forces returning to their bases and only 50 additional officers in northern Paraguay to confront the group.