Mexican authorities say the men were shot while off duty in Guadalupe, a suburb of Monterrey. They were riding a motorcycle when gunmen in a vehicle opened fire.
U.S. Embassy spokesman Alexander Featherstone confirmed the men were assigned to guard U.S.
diplomats. He said U.S. officials were offering support to Mexican authorities investigating the apparent homicides.
"They were members of the extended family. We are deeply saddened by the deaths," Featherstone said.
He did not provide further details.
Mexican authorities say the officers guarded the U.S. consulate in Monterrey.
Violence has been rising in Monterrey to levels rarely been seen in Mexico outside border cities such Juarez, Tijuana and Nuevo Laredo Drug-related murders this year are on pace to double last year's and triple those of the year before.
Elsewhere in northern Mexico, authorities say soldiers killed six gunmen who had apparently set up a highway roadblock.
Mexico's Defense Department says the soldiers came under fire Friday when they responded to a tip that armed men were blocking a highway in a rural area of Zacatecas state. It says the troops returned fire and killed six suspects.
The soldiers recovered seven rifles and four vehicles.
Mexican drug cartels have been operating in the area and have been known to set up roadblocks to steal vehicles and rob travelers.
Three people were also found killed Friday in the resort city of Acapulco, where drug violence has also been rampant in recent months. In addition a police officer who had been collecting weapons from drug gangs there was shot by assailants, who made off with at least two rifles.
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