Nine bodies were found in a bloody pile in the border city of Nuevo Laredo, across from Laredo, Texas, Mexican authorities reported.
A Tamaulipas state official said Thursday the dead include five women and four men. The official was not authorized to be quoted by name and spoke on condition of anonymity.
The official said a hand-lettered sign was left atop the bodies, which were found in front of a house near a border bridge. Such messages are frequently left by drug cartels as warnings to rivals.
"This is not a joke, nephew," read the sign, according to photos published in local media.
The state prosecutors' office said in a statement it was investigating the killings.
Nuevo Laredo has long been dominated by the Zetas cartel, which has splintered into factions following the arrest or killing of top leaders.