The bodies showed signs of beating and torture and "we believe they were members of a criminal gang," the official said, declining to be named.
Photographs showed the bodies strung, with their hands tied, along a wide road bridge in the border city in northeast Mexico, across from Laredo, Texas, in a typical sign of score-settling between drug gangs.
A large banner hung nearby with a message alluding to gang disputes.
There were scenes of panic as road users first saw the bodies, which police reports said had been strung up early Friday.