BOGOTA, COLOMBIA, JUNE 14 -- The Medellin cocaine cartel set off a powerful truck bomb in Colombia's second city today, killing four persons and wounding 20, in what police said was retaliation for their killing of one of the cartel's top hitmen.

The Elite Corps of the National Police, acting on an informer's tip Wednesday night, found and killed Jhon Jairo Arias, reportedly a close adviser of cartel leader Pablo Escobar and leader of one of the cartel's most deadly bands of hired teenaged assassins.

Police said Arias was killed trying to escape his plush apartment after it was surrounded.

"Arias was one of the most important people in the Medellin cartel," said Gen. Miguel Gomez Padilla, director of the National Police, in a news conference today. "He was involved in many of the attacks in Medellin in recent days and is wanted for many homicides, including some of the most notorious murders in the country."

Gen. Octavio Vargas, chief of police operations, said Arias was suspected of helping plan the assassination of two presidential candidates earlier this year.

In recent weeks, Medellin, a city of 1.8 million residents, has been in virtual chaos as the war rages among police, the cartel's private army and armed bands of common criminals.

Human rights groups and officials estimate that people are being killed at a rate of 35 a day, often unidentified young men bearing signs of torture. Human rights workers attribute the deaths to all sides in the conflict. About 170 policemen have been killed in Medellin this year.

According to a police statement on today's bombing, the four persons were killed when a truck carrying about 250 pounds of dynamite exploded in front of a police station in a plush section of Medellin at about 10 a.m. -- when most of the country was watching Colombia play in the World Cup.

The police said that when officers tried to stop the truck for a routine search, the driver activated the bomb, killing himself, a policeman and two civilians, as well as destroying 20 vehicles. The statement said the bombing was in retaliation by cartel for the death of Arias.

Gomez Padilla said the informer's tip was the first direct result of a new police campaign to get Medellin residents to help capture leaders of the cocaine cartel. The general said the informer would receive a reward.

Last week, police distributed 20,000 wanted posters, mostly in Medellin, bearing recent pictures of 12 of the most-wanted leaders of the cartel. Police offered a $200,000 reward for turning any of them in, and promised protection for informers.

"Denounce them," the poster said. "Do not become the next victim."