The cartels have also militarized. Bit of a vicious cycle we’re in.
Cartel leadership have openly stated that any such thing would be the height of counterproductive stupidity on their part due to the overwhelming response that it would elicit.
The last confirmed incident was in the 90s. There was an incident in Texas a couple of days after Trump was inaugurated but this seems a little lacking in concrete details or threat: across the river, no video evidence, no injuries, no bullets recovered, no suspects, no identification, just a broad "description" of the shooters as "likely cartel members".
The cartels have long been wary of US armed response to their actions, and the militarization of the police in the US, while certainly a further deterrent, is much closer to a deterrent against CIVILIAN misconduct and works insanely well as a chilling effect against protestors and unruly gatherings. Yay America!!!!