Four police officers were killed and 42 others injured on Saturday when a bomb exploded outside a police station in Colombian city of Barranquilla, police said.The Colombian National Police Director General Jorge Hernando Nieto termed the attack "demented" and said: "I express all our solidarity ... with the families of the four police who have died thus far and with the more than 40 police who suffered some type of injury." The attack occurred when officers were lined up during a shift change, Efe quoted the commander of that city's Metropolitan Police force, General Mariano Botero, as saying.He said that an explosive device had apparently been placed in one of the station's walls.An individual activated the bomb by remote control, according to Botero, who said the station's central patio was left in ruins, with debris and police helmets scattered everywhere.The attack was initially thought to have been a grenade blast, but a suspect had been arrested nearby with a notebook and communications gear.Authorities are trying to determine whether the explosion was related to a separate, nearly simultaneous attack near Barranquilla on an armored truck used to transport valuables.One guard was killed and two others were wounded in that attack, in which nothing was stolen.Barranquilla Mayor Alejandro Char said earlier that he believed organised-crime elements had carried out the attack on the station in retaliation for blows the police has dealt drug traffickers in Barranquilla and other parts of Colombia's Caribbean coast.