The note, which was purportedly written by a powerful drug cartel, apologized for the attack last week that left three people dead, and offered up five men it said were responsible.
Many noted the coordinated action after four Americans were kidnapped, and two killed, in a country where crimes against Mexicans get far less attention.
Un grupo de amigos viajó a Tamaulipas el viernes, pero a las pocas horas fueron secuestrados por hombres armados. Dos de ellos murieron antes de que los otros dos fueran rescatados el martes.
The four came under fire shortly after entering the city of Matamoros from Brownsville, Texas. The F.B.I. is offering a $50,000 reward for their return and the arrest of the kidnappers.