Police in Texas have been left stumped by messages painted onto two billboards, alongside mannequins dressed in suits and hung from nooses.
The messages, which were daubed in white paint on two plaques in central and east El Paso, were brought to the attention of officers by motorists early Thursday morning.
One message read “Plata o Plomo”, meaning “silver or lead” in Spanish; a Mexican phrase that could mean “take the money or be killed”, the El Paso Times reported.
The other billboard bore the phrase: “Dying for drugs”.
Both billboard companies said the messages were vandalism and not paid advertisements – they have since been removed.
But it remains unknown who is behind the graffiti or who the intended recipient of the words is.
Sgt. Chris Mears, a spokesman for the El Paso Police Department, said in a statement: “This symbol (plata o plomo) has historically been used by Mexican drug cartels to threaten or intimidate Mexican citizens, business owners and government officials. However, we have never experienced this in El Paso.
"The investigation is on-going, but we do not have any information to suggest this was done to target any individual person or business at this time."
Drug cartels sometimes hang banners in Mexican cities with graphic threats and have even hanged victims from overpasses, but such public threats are virtually unheard of on the US side. El Paso has maintained a crime rate that is among the lowest in the country, even during recent years when Ciudad Juarez, across the border, suffered a bloody cartel battle.
"Whoever did this went through a lot of work to get this accomplished," said Phil Jordan, a former Drug Enforcement Administration agent who ran the El Paso Intelligence Centre.
"This is possibly a message to someone who hasn't cooperated with the cartels. But even if it's a hoax, something like this is going to make the El Paso population uneasy, given that the city is not far from the killing fields of Mexico."
Additional reporting by Associated Press