Attackers have killed at least 29 people - decapitating most of the victims - on a ranch in a part of northern Guatemala plagued by drug cartels.
The massacre took place on Sunday in the town of Caserio La Bomba in Peten province near the Mexico border, said National Civil Police spokesman Donald Gonzalez. Among the dead were two women and two children.
It is one of the worst massacres since the end of Guatemala's 36-year civil war in 1996.
Mr Gonzalez said police were investigating whether the attack was related to Saturday's killing in Peten of Haroldo Leon, the brother of alleged Guatemalan drug boss Juan Jose "Juancho" Leon.
"Juancho" Leon was killed in 2008 in an ambush that Guatemalan authorities blame on Mexico's Zetas drug cartel, which has increasingly wrested control of the drug trade beyond Mexico, at times by eliminating the competition. Ten others were killed in the 2008 attack.
Guatemalan authorities said police and soldiers were searching the area for the unidentified attackers and did not offer a motive for the killings.
"This is a terrible event that we must clarify and investigate regardless of the consequences, whoever is the author of this massacre," said Guatemala prosecutor general Claudia Paz y Paz.
Guatemala has become a major shipment point for drugs heading north to the US.
In February the government lifted a two-month-long state of siege that it had declared in Alta Verapaz province, which neighbours Peten province, during which security forces were sent to quell drug-related violence.
The state of siege gave the army emergency powers - including permission to detain suspects without warrants - and resulted in the arrest of at least 20 suspected members of the Zetas gang - a group of ex-soldiers who began as hitmen for Mexico's Gulf drug cartel before breaking off on their own, quickly becoming one of Mexico's most violent gangs and spreading a reign of terror into Central America.