CIUDAD VICTORIA, Mexico (AP) — Gunfire broke out across the Mexican border city of Nuevo Laredo early Monday, prompting the cancellation of classes and an advisory from the U.S. Consulate to shelter in place.

President Andrés Manuel López Obrador said the shootings came in response to the arrest of a cartel leader, but did not elaborate. The city has long been dominated by the Northeast cartel, an offshoot of the old Zetas gang.

The U.S. Consulate in Nuevo Laredo cancelled visa appointments for the day.

"There is an emergency situation in Nuevo Laredo following an arrest operation. Reports of gunfire in multiple locations in the city. Shelter in place and standby for further information," the consulate said.

A military officer who was not authorized to be quoted by name said the suspect was a leader of the Northeast cartel's infamous "Troops of Hell" squad of killers. That squad acts as an enforcement arm of the cartel and has often tangled with law enforcement and military forces.

The officer identified the detainee as Heriberto Rodríguez Hernández, known by his nickname "Negrolo" or "Negro 35." Negro means Black in Spanish, but it is unclear if the nickname refers to his race.

Bus services stopped running, Mayor Carmen Lilia Canturosas reported, "due to the situation in the city." She reported "risk situations at different points in the city."

In videos posted on social media, bursts of gunfire could be heard in Nuevo Laredo in the pre-dawn hours.

By mid-day Monday, the mayor tweeted "the situation in the city is starting to normalize."

Many cartels in Mexico react to the arrest of leading members by creating chaos, either to avoid the detention or transfer of suspects, or to pressure authorities to release them. López Obrador said the suspect had already been taken to Mexico City.

It was the second time this year that Nuevo Laredo has been hit by cartel violence after a capo's arrest.

In March, gunfire and burning vehicles in Nuevo Laredo led U.S. officials to close the U.S. Consulate and briefly shut down border crossings into Laredo.

Those events in March followed the arrest of another leader of "The Troops of Hell," Juan Gerardo Treviño, a nephew of imprisoned Zetas leader Miguel Angel Treviño.