The United States diplomatic post in Nogales, Sonora released a notice Friday night about an unnamed danger. American officials limited their own work to daylight and told citizens not to go there, to take cover if trouble arose, and to update family members.

Mexican authorities in Sonora said they knew of no such problem and contacted the post, which verified the notice without explaining it. Local forces said they would continue watching events.

The notice arrived as Washington had posted comparable cautions for Baja California and Michoacan, where one earlier case involved a cartel targeting American officials and another involved intelligence of a possible cartel strike on police stations.