(CNN) -- There are dozens of murders recorded in Mexico each month, but it was a single one that set off a succession of skirmishes that has turned the northeastern state of Tamaulipas into a battle zone.
For weeks, the border city of Reynosa, Tamaulipas, and surrounding towns have been the scene of a spike in violence and fear that have included high-caliber shootouts in broad daylight.
The killing of one cartel member by another cartel sparked the surge in violence, U.S. authorities said, a reminder of how the micro and the macro are linked in drug trade and the communities where it operates.
After an increase in violence earlier this month, Tamaulipas has quieted down a bit. But the violence may have just shifted west to the neighboring state of Nuevo Leon, where two college students were killed after getting caught in a skirmish between authorities and traffickers.
The violence is the result of a new rivalry between former partners, the Gulf cartel and a group known as Los Zetas, said Will Glaspy, head of the Drug Enforcement Administration's McAllen, Texas, office.
Until spring 2008, the Zetas were part of the Gulf cartel, a group of Mexican special forces defectors who worked as the cartel's ruthless enforcement arm.
"The role of the Zetas evolved from hired guns to their own independent criminal organization," Glaspy said, adding that tensions between the two groups increased in the past six months.
The tipping point was the unannounced visit of a Zeta higher-up to Reynosa, where he was confronted and killed on orders from the Gulf cartel, Glaspy said.
Glaspy did not identify the victim, but others, including the global intelligence company Stratfor, say the man was Sergio Mendoza Pena, a top Zeta leader.
Mendoza Pena, described as the right-hand man of the Zetas' No. 2 leader, Miguel Trevino Morales, was killed after an altercation with one of the top Gulf cartel leaders, Eduardo Costilla Sanchez, according to Stratfor.
The Zetas gave the Gulf cartel an ultimatum to turn over the killers, but the deadline came and went, Stratfor technical analyst for Latin America Alex Posey said.
Since then, it's been a tit-for-tat battle that spilled into the streets of northeastern Mexico, Posey said.
Fear of reprisal by the drug cartels has led to a virtual media blackout in the region, making it difficult to sort truth from rumors.
Rumors have swirled about out-of-control violence that has left dozens of civilians dead in the cross fire and a helicopter shot out of the air.
But the Tamaulipas government insists that these reports are just that -- rumors.
Social networking, particularly Twitter, has emerged as a source of information on the violence.
"There is some degree of truth to the information you find on Twitter, but that can't be your only source of information," Posey said.
A message published by the U.S. consulate in Monterrey, Mexico, earlier this month shed some light on the fallout of the Zetas-Gulf cartel fight.
"There have been numerous confirmed reports of deadly gunbattles taking place in and around the cities of Nuevo Laredo and Reynosa in the neighboring state of Tamaulipas and in small towns of Nuevo Leon that are north and east of Monterrey," the statement said.
The document also confirmed instances where the dueling cartels were setting up vehicle checkpoints on major highways that link Monterrey, in the state of Nuevo Leon, to the border.
"Both sides are amassing numbers of personnel in preparation for an ongoing conflict," Posey said.
There is also evidence that each side is recruiting drug trafficking organizations from other parts of the country to take sides and join the battle, Posey said.
The fact that things have quieted down a little could signal that things won't get out of control.
"We have gotten intelligence briefings that the warring factions are now attempting to negotiate and try to verbally settle their disputes rather than having shootings in Reynosa and other cities," said Hidalgo County Sheriff Lupe Trevino. Hidalgo sits on the Texas border, across from Tamaulipas state.
Glaspy, the DEA leader, said his office had heard talk of negotiations, but had not confirmed it.
What is for sure, he said, is that enforcement efforts by the Mexican military continue, as do trafficker-on-trafficker skirmishes.
"Things are quieter than a month ago," Glaspy said. "Where it's going to lead from now, I can't tell you. What we hope is that cooler heads will prevail."