Gunmen entered a busy Nuevo Laredo plaza and laid out the nude bodies of four slain men next to their severed heads on a sheet late on Friday, police said.
“We got a call and members of several police forces rushed to the site to investigate,” a municipal police officer who declined to give his name said on Saturday.
The sheet had a message apparently signed by members of the Gulf drug cartel that was aimed at a rival gang. The officer did not divulge the content of the message.
The gunmen arranged the macabre scene in full view of horrified pedestrians in the busy downtown area, the officer said.
Nuevo Laredo, population 350,000, is one of the busiest commercial crossing points of goods from Mexico into the US. It borders the US state of Texas.
The Gulf cartel is engaged in a bitter war to control the lucrative drug smuggling routes into the US with the Zetas, its former allies, a group made up of former military commandos.
Meanwhile, a judge has authorized 40 days of detention for an alleged Zetas member suspected in the killing of a US immigration agent.
The Attorney General’s Office said the judge’s order will allow federal investigators to conduct a more thorough investigation of Julian Zapata Espinoza’s alleged involvement with the drug cartel.
Zapata Espinoza is the main suspect in the Feb. 15 killing of US Immigration and Customs Enforcement agent Jaime Zapata.
The office said in a statement on Saturday that the judge ordered that eight other people arrested along with Zapata Espinoza also be held for 40 days. Two of them are suspected of taking part in the attack, which also wounded US immigration agent Victor Avila.
More than 36,400 people have been killed in Mexico’s drug wars since 2006, when the government of Mexican President Felipe Calderon deployed some 50,000 soldiers and police across the country to crack down on the drug trade.