A car exploded Thursday in the northern city of Monterrey as a military convoy chasing a suspicious vehicle was passing it. No one was injured, authorities said.
Police in Nuevo Leon state, where Monterrey is located, are trying to determine if the car was loaded with explosives or if assailants threw grenades at it, said a military official who spoke on condition of anonymity because of the ongoing investigation.
Photographs of the scene showed an almost-disintegrated small car and the damaged metal door of an auto repair shop. The explosion caused no other reported damage.
The troops were on a routine patrol in Monterrey when they spotted a suspicious car carrying several men and gave chase.
The industrial city of Monterrey has seen a spike in drug violence since early 2010, when the Gulf drug cartel and the Zetas began a fierce fight for control of the city and its metropolitan area.
Drug traffickers first used explosives against authorities in July 2010, when a car bomb aimed at federal police officers exploded in Ciudad Juarez, killing three and wounding nine. The following months, three other car bombs were detonated in Ciudad Victoria, capital of the border state of Tamaulipas.
Also Thursday, a federal judge sentenced four military officers to more than 29 years in prison for giving drug traffickers information on police operations, the Attorney General's Office said in a statement.
The four soldiers, three lieutenants and a captain were arrested in June 2009 as part of a widespread corruption probe known as "Operation Clean House."
The officers were linked to drug lords Ismael Zambada and the Beltran Leyva brothers, the statement said.
They were giving them "information that allowed them to evade justice and operate with impunity."
The military officers are Capt. Jose Manuel Reyes, and Lts. Ricardo Santos Vazquez, Jaime Guatemala Nino and Francisco Jimenez Garcia. Each received a sentence of 29 years, four months and 15 days in prison, federal prosecutors said.
They also have to pay fines ranging from $13,600 to $122,000.
Federal prosecutors didn't say where the officers were working while allegedly collaborating with drug traffickers or how long they allegedly passed information to them.
Zambada is one of the leaders of the Sinaloa drug cartel, Mexico's most powerful. The Beltran Leyva brothers were once part of the same cartel but they split after the 2008 arrest of Alfredo Beltran Leyva.