The newspaper said police believe a motorcycle passenger lobbed a fragmentation grenade which exploded outside the newsroom on Monday evening as three journalists and a designer were finishing Tuesday's edition. The blast damaged the building but caused no injuries.
El Informador is a family-owned daily based in the port city of Santa Marta, in northern Magdalena Department. It was unclear why it was attacked.
Reporter Rafael Rodríguez Santana, who was in the office during the attack, said no one had claimed responsibility. Rodríguez said the newspaper had recently published articles on the capture of alleged paramilitary combatants and the detention of public officials accused of corruption.
Joana Romero, an editor, said the staff had received no threats and journalists were surprised by the attack.
CPJ Executive Director Ann Cooper said, "We condemn the attack against El Informador and call on Colombian authorities to conduct a thorough and prompt investigation to find those responsible."