MEXICO CITY — Mexican authorities on Tuesday reported the discovery of 28 more bodies in a northern state, bringing to 116 the number of dead unearthed since officials began investigating mass kidnappings of bus passengers.

As horror mounts over the savagery in Tamaulipas state, federal officials said they had sent in more troops and would carry out “”constant monitoring”” of highways in the violence-ravaged border state.

The government of President Felipe Calderon has poured troops into Tamaulipas after previous episodes of grisly violence. But nothing has quelled bloodletting by drug-trafficking gangs that are effectively in control of big swaths of the state.

Mexico’s interior minister, Francisco Blake, said criminal gangs were acting out of “”desperation.”” Officials have arrested 17 people in connection with the graves.

“”Organized crime, in its desperation, resorts to committing extraordinary atrocities that we cannot and should not tolerate,”” Blake said after meeting with Tamaulipas Gov. Egidio Torre Cantu in Mexico City.

Before Tuesday, a series of mass graves found within a week in rural San Fernando had yielded 88 bodies, amid reports that gunmen were stopping buses and seizing passengers.

Authorities blame the Zetas gang, which was accused last year of kidnapping and killing 72 migrants from Central and South America in the same area after trying to force them to work for the group.

Mexican authorities have not spelled out any motive for the bus attacks on a highway that leads to the U.S. border, 90 miles to the north. The region is traversed by thousands of Mexican and Central American migrants headed for work in the U.S.

U.S.-bound migrants crossing Mexico are often targets of robbery or attempts to extort money from loved ones in the U.S. or back home.

A chilling byproduct of the ruthless drug war in Mexico that has killed more than 34,000 people in four years is the disappearance of thousands of people, as the Los Angeles Times reported last month.

Officials had said they believe most of this month’s victims are Mexican, but only a few have been identified so far. The Guatemalan government said at least one of the victims is from Guatemala.

Coroners have taken DNA samples from 72 victims in an effort to check if they match those of dozens of people reported missing from the central states of Guanajuato and Queretaro who were reported missing after heading north. A U.S. citizen is believed among those seized from buses.