Six severed heads found alongside ominous note on busy street in Mexico

It has sparked a major investigation

The discovery was made early Tuesday morning by authorities in an area which is usually peaceful and not at all associated with Mexico's cartel violence.

The road on which the severed heads were found links the central states of Puebla and Tlaxcala.

Police have not yet found a motive behind the killings. They've also not disclosed any information regarding which criminal groups could be linked to the violent incident.

It was reported by local media that a blanket was left at the horrific crime scene with an ominous message warning rival gangs. It was allegedly signed by a group called "La Barredora" ("The Sweeper"), per the Mirror.

La Barredora is a criminal gang that operates in the western state of Guerrero, however, there's no clear proof on whether this cartel was behind the attack.

Federal authorities are yet to issue an official comment on the killings, however, the Tlaxcala Prosecutor’s Office has said the heads found in Tlaxcala were those of men.

Aside from drug-trafficking, the region is also known for fuel smuggling ("huachicoleo"), which generates billions of dollars yearly for the gangs behind the illegal activity.