In a bold public display of the gang violence sweeping across northern Mexico, residents in the border city of Nuevo Laredo awoke at dawn Friday to find nine corpses of men and women hanging from a bridge at a busy intersection just a 10-minute drive from Texas.

A few hours later, authorities discovered 14 headless bodies wrapped in plastic bags, stuffed into a sport-utility vehicle in front of a Mexican customs agency. The 14 heads were later placed in plastic-foam coolers and left by armed men on a crosswalk beside the city hall, according to the attorney general in Tamaulipas state.

Residents accustomed to violence in Nuevo Laredo erupted in fear and disgust on social media networks. One tweet read: “We have no law in Nuevo Laredo. Welcome to the Jungle!” A car bomb exploded in front of a police station last month, followed by a gun battle between Mexican soldiers and gangsters.

A Web site devoted to news about narco-violence published photographs of the nine victims — five men and four women — swinging from the bridge, the corpses bloody and bearing marks of torture. Some had their pants pulled down to their ankles.

There was a banner hung beside the bodies on the bridge, and its profanity-laden message boasted that “in this way I am finishing you all off.” It also said that one victim “cried like a woman giving birth.”

It was unknown who left the bodies hanging from the bridge or whether the 14 decapitated corpses found later were a response. Local police and state security officials reported no motives or arrests.

“It appears there is a really awful fight going on for the control of Nuevo Laredo,” said Raul Benitez Manaut, a drug policy scholar.

The city is an important gateway for smuggling drugs and people north to the United States, and for shipping bulk cash and weapons south to Mexico. Nuevo Laredo is a battleground between the Gulf Cartel and its former enforcers, the Zetas.