McAllen, Texas ? An already tense situation along the border worsened Saturday amid an internal FBI memo warning that a ruthless Mexican drug cartel could be plotting to kidnap and murder U.S. federal law enforcement agents.

Although the plot specifically targets two unidentified agents of the FBI, the bulletin warns that “due to the nature of this immediate threat, all law enforcement personnel are being cautioned to ensure appropriate measures are taken as well as to keep a high degree of vigilance,” the bulletin stated.

The threats originate from members of the Osiel Cardenas-Guillen cartel, the bulletin said, “an extremely violent Mexican drug trafficking organization commonly referred to as the Gulf Cartel. This cartel is alleged to have recently assembled over 250 armed men near the Mexican border town of Matamoros, Tamaulipas across from Brownsville, Texas.

The gulf cartel has a presence in 13 Mexican states, operating with a paramilitary arm, integrated from ex-members of the Mexican armed forces known as the Zetas, who remain loyal to Cardenas, one of the country’s most powerful cartel bosses jailed in a Mexican federal prison.

The consequences of this and other recent warnings were evident along the 2,000-mile U.S. border with Mexico, as shoppers again stayed away from popular border destinations, from Ciudad Juarez to Reynosa.

In Brownsville, a group of Texas lawmakers who had been scheduled to meet with the Tamaulipas governor and mayors rearranged their agenda so their Mexican counterparts could instead meet them on the U.S. side.

“Any threat from the cartels or splinter groups has to be taken seriously by law enforcement,” said Phil Jordan, former director of the El Paso Intelligence Center. “These organizations operate at almost the level of terrorist. Their ruthlessness is only second to terrorists.”

Recent violence along the U.S.-Mexico border and a rash of kidnappings of U.S. citizens have led to a travel alert. Pablo Cisneros stands in front of a poster of his daughter Brenda, left, and her friend Yvette Martinez in Laredo, Texas. The pair disappeared Sept. 17 from Nuevo Laredo, Mexico.

The warning follows a crackdown by the Mexican government on the Gulf Cartel, which calls home this region stretching from Nuevo Laredo to Reynosa and Matamoros.

Two weeks ago, federal authorities raided La Palma prison in the state of Mexico to take back control of the institution from jailed kingpins — chiefly Benjamin Arellano Felix, the head of the so-called Tijuana Cartel, and Cardenas, the head of the Gulf Cartel.

The extreme violence along the border, plus a rash of kidnapping of U.S. citizens — 25 in the last six months in Nuevo Laredo alone — led to a travel alert by the State Department, accompanied by a letter from U.S. Ambassador Tony Garza.