A bomb set off by suspected rebels ripped through a shopping center in northeastern Colombia on Wednesday, killing seven people, injuring at least 20 and setting the complex on fire.

Television images showed shocked survivors wandering around the shopping stalls, blackened Ash Wednesday marks still on their foreheads.

Government officials said the attack in Cucuta, on the border with Venezuela, was part of an attempt by guerrillas to bring the nation's long-simmering war to Colombia's cities.

Defense Minister Martha Lucia Ramirez said the urban offensive is a response to government offensives in the countryside. "Unfortunately, that is the risk we have to run," she said.

After the bombing, President Alvaro Uribe appointed a police general to find rebel infiltrators in Cucuta government offices. The state news agency identified the prosecutor's office as one government agency where people got jobs because of pressure from rebels.

"The police are conducting an internal investigation and will do everything necessary in Cucuta and North Santander [state]," Uribe said, according to the State News Center agency.

Cucuta Mayor Manuel Guillermo Mora said seven people were killed in the blast.

Police Gen. Luis Alfredo Rodriguez said the National Liberation Army, or ELN, was responsible for the attack. The bomb was left next to a car in the basement parking lot, police said.

Firefighters, rescue crews and police officers hustled through the smoke into the shattered complex as frightened passers-by looked on.

Family members of shopping center employees gathered nearby waiting for word of their relatives.

The ELN and the nation's largest rebel group, the Revolutionary Armed Forces of Colombia, are battling outlawed paramilitary groups for control of Cucuta. The city has one of the highest murder rates in Colombia.

Colombia is torn by a 38-year civil war that pits the leftist rebels against the government and right-wing paramilitary groups. About 3,500 people, mainly civilians, are killed in the fighting each year.