MEXICO CITY (AP) — Halloween is gaining ground in Mexico, but in a country ravaged by drug cartel violence there are real fears about ghosts, ghouls and skeletons walking the streets.

The concern is not so much competition for Day of the Dead, traditional, grown in Mexico celebrations, which take place on Friday and Saturday without a hitch in cemeteries across the country this year.

Rather, in at least three violence-plagued Mexican cities, authorities have warned residents about masks, which are frequently used by Mexican cartel gunmen to hide their identities.

In the northern cities of Tijuana, Culiacan and Hermosillo, authorities warned residents not to stay out late and adults not to wear masks.

Tijuana’s local government secretary Arnulfo Guerreo on Thursday announced special security measures for the city’s “Operation Halloween,” which involved hundreds of police officers guarding the downtown Halloween-style celebration.

“It’s already in the regulation that you can’t wear masks,” said Arnulfo Guerreo, secretary of Tijuana’s local government, a rule that largely refers to the ski masks favored by gunmen.

“That doesn’t mean don’t wear suits, it’s just the issue of the masks, it’s something that helps protect us and our families,” Guerrero said. Authorities later told local media that police would use their judgment in issuing fines and that the rule would only apply to adults.

Based on social media videos of Thursday night’s festivities on Tijuana’s Boulevard de la Revolucion, the rule appeared to have gone largely unenforced: Adults could be seen dressed in masks and costumes from Beetlejuice, Scream, Friday the 13th and other popular franchises of horror films.

Also just before Halloween, the police chief of the northern state of Sinaloa, which has been devastated for weeks by infighting between factions of the Sinaloa drug cartel — gave a more grim warning.

“The recommendation is not to stay out too late, not to go trick-or-treating or wear disguises,” said state police chief Gerardo Mérida, adding the chilling phrase “at night, all cats are black.”

Although it might sound like a terrorist movie curse, it’s an expression used in Spanish to mean “in the dark, it’s easy to mistake something for something else.”

In a city where trigger-happy military forces have already killed a bystander while searching for cartel suspects, it was a chilling warning.

Brigades of police and militarized National Guard officers were sent to patrol the streets of Culiacan, the state capital, on Thursday night, and fortunately, Mérida told local media on Friday that Halloween was relatively peaceful for the battered city, with only a few reports of gunfire.

Hermosillo, the capital of the neighboring state of Sonora, also urged residents to go public without Halloween masks.

Elsewhere in Mexico, the Day of the Dead was celebrated on Friday to mark those who died in childhood; Saturday is dedicated to those who died as adults.

Observations included whole families cleaning and decorating graves, which were covered with orange marigolds. Both at cemeteries and at home shrines, relatives would light candles, put out offerings of their deceased relatives’ favorite foods and drinks.