Banners similar to those left by Mexican drug cartels appeared Thursday threatening to attack schools in three communities outside the northern Mexico city of Monterrey.
Similar banners and messages left near schools in the southern city of Acapulco led dozens of grade schools to close in late August.
But there was no immediate indication that the banners left in communities in the township of Santiago on the outskirts of Monterrey would affect classes there.
Santiago Mayor Vladimiro Montalvo Salas said he asked state and federal authorities to step up security in the township, but urged parents to continue sending their children to school.
Montalvo Salas said he hoped the banners were just a bad joke.
He said state police had taken down the banners, adding that it was unclear whether they contained any demands for protection payments as the messages in Acapulco did.
Drug gangs are known to operate in and around Monterrey. Last year, Montalvo Salas' predecessor as mayor was killed.
Santiago was once considered a quiet, bucolic weekend getaway for Monterrey residents, drawn by the town's quaint colonial-style streets. But the violent Zetas cartel began operating in the area around 2010.
In Acapulco, teachers agreed Thursday to gradually reopen dozens of grade schools that have missed classes, some since late August. The reopening is scheduled to begin Monday.
Arturo Martinez, the spokesman for the government of Guerrero state, where Acapulco is located, said Gov. Angel Aguirre had signed an agreement with leaders of local teachers unions pledging increased security for schools that were targeted by threats and extortion demands.
Estimates vary on how many schools were affected. Edith Zurita Petatan, spokeswoman for striking teachers, claimed hundreds of schools shut down in the seaside city of almost 800,000 people. The state government said only about 50 schools closed.
Elsewhere in Mexico on Thursday, the navy reported that marines had killed suspected cartel gunmen in a shootout in Veracruz state on Mexico's Gulf coast.
The marines were responding to a complaint about armed men in the area around the city of Cosamaloapan when the gunmen opened fire on the patrol, the navy said.
Marines reported finding three rifles, drugs and a sport utility vehicle at the scene of Thursday's confrontation.
The Zetas cartel also operates in that area.