The cartel uses Laredo as a base to gather intelligence and weapons and to launch strikes against rivals, the authorities say. Cartel members also want to take advantage of the protection offered by the ''softer justice'' available on the U.S. side.

These conclusions are based on recent large seizures of powerful weapons and homemade grenades, police scanners and cash discovered in Laredo safe houses over the last two months, as well as intelligence that U.S. authorities have gleaned from the growing number of Mexican nationals in local jails, law enforcement officials said.

Some of these inmates are members of the Gulf drug cartel or the Sinaloa cartel, which some officials say have operatives in North Texas. Several inmates are being held in connection with cartel activities across the border in Nuevo Laredo, Webb County authorities and defense lawyers said.

''You have to look at it from the eyes of the bad guys,'' said Fred Burton, a former U.S. State Department agent and current vice president of counterterrorism for Austin, Texas-based Stratfor, an information-gathering company that draws on U.S. intelligence for its reports. ''Laredo represents a safe haven, a logistical area to hunker down and strategize future hits.''

The city has become ''a cesspool of cartel activity,'' noted a U.S. law enforcement official.

''This place is becoming a fertile staging ground for them, and it's happening right under our noses,'' the official said. ''We don't have the same murder rate of Nuevo Laredo, but we're increasingly witnessing the same type of violence here.''

Another reason for operating on the U.S. side, authorities say, is so-called ''softer justice.'' Cartel operatives in the Laredo-Nuevo Laredo area know that if they are caught north of the Texas-Mexico border, they likely will go to jail and be able to fight extradition through lawyers, authorities said. If they are apprehended south of the border, however, they are more likely to face execution in jail by rivals or corrupt local and state police in Nuevo Laredo, law enforcement experts said.

Laredo officials have played down the occurrence of any spillover violence from Nuevo Laredo. City officials note, for example, that the city has experienced just two murders this year, compared with 46 across the border.

Those two killings, they add, have no connection with events in Nuevo Laredo, where just Tuesday a daytime shootout left two state police officers and two others dead.

Presidential spokesman Ruben Aguilar said this week that drug traffickers appeared to be behind that incident. The shootings apparently stemmed from internal struggles for control of the drug cartels, Aguilar said. He added that federal prosecutors may take charge of the case.

Laredo officials say their city has remained relatively free of drug violence. Last year, 20 slayings were recorded in Laredo, and ''one, maybe two of those killings could have been connected to crime in Nuevo Laredo,'' said Juan Rivera, a spokesman for the Laredo Police Department.

''But there really is no proof at this time. It's like the old saying goes: 'Whatever happens in Vegas stays in Vegas.' The same can be said for Mexico. What happens in Mexico stays in Mexico,'' Rivera said.

Some American and Mexican law enforcement officials take a different view. Testifying before Congress last month, Webb County Sheriff Rick Flores said as many as nine of the 20 killings were linked to cartel violence in Nuevo Laredo.

''It's impossible for you to run a criminal organization without having people on the other side,'' Flores said in an interview.

''We do have people on this side working with the cartels on the other side, and that's definite.''