An alleged plot by a Mexican drug cartel to blow up a dam along the Texas border — and unleash billions of gallons of water into a region with millions of residents — sent American police, federal agents and local disaster officials scrambling last month to thwart such an attack, authorities confirmed Wednesday.

Whether the cartel, which is known to have stolen bulk quantities of gunpowder and dynamite, could have taken down the five-mile-long Falcon Dam may never be known.

But it may have been derailed by a stepped-up presence by the Mexican military, acting in part on intelligence from the U.S. government, sources said.

The warning was based on what the federal government contends were “serious and reliable sources” and prompted the Homeland Security Department to sound the alarm to first responders all along the South Texas-Mexico border.

Mexico's Zeta cartel was planning to destroy the dam not to terrorize civilians, but to get back at its rival and former ally, the Gulf cartel, which controls smuggling routes from the reservoir to the Gulf of Mexico, Zapata County Sheriff Sigifredo Gonzalez and others familiar with the alleged plot said.

Destroying the dam, however, also would have flooded large areas of agricultural land, as well as significant parts of a region with about 4 million border residents in Texas and Mexico.

The dam along the Rio Grande was finished in 1954 as part of a joint U.S.-Mexico project to collect water for flood control, hydroelectric power and water for drinking and agriculture.

Gonzalez's agency was among many that responded, as did the U.S. Border Patrol, the Texas Department of Public Safety and even game wardens, who put more boats on the water.

Citing security concerns, neither Homeland Security nor DPS commented.

“We trust that DPS and their federal and local law enforcement partners are constantly collecting intelligence and monitoring all threats to Texas and taking the appropriate action to protect our citizens from those who would do us harm,” said Gov. Rick Perry's deputy press secretary Katherine Cesinger.

Law enforcement officials huddled at the dam, located near Rio Grande City, to discuss the threat as well as how to thwart it, said an officer who attended the meeting.

Officers interviewed by the Houston Chronicle gave the warning varying degrees of credibility. They noted that among the Zetas' ranks are Mexican military defectors who were trained in special forces tactics, including demolition.

In response to the threat, cameras were set up along the dam, which has six 50-foot-tall steel gates, and authorities hid in brush.

A Mexican military spokesman, who spoke on condition of anonymity, said he hadn't heard of any threat to the Falcon Dam and expressed doubt the Zetas would try such an attack.

“This isn't the way these groups operate. They have never attacked installations like that,” he said. “There are no precedents of this kind of action.”

Rick Pauza, a spokesman for Customs and Border Protection, in Laredo, said the port of entry at the dam had been at a heightened state of alert because of ongoing violence in Mexico.

With handbills and bullhorns, members of the Zeta cartel are said to have warned the civilian population on the Mexican side of the river in the vicinity of the dam to get out of the area, according residents and intelligence information from law enforcement officials.

A longtime border law enforcement official said the concerns were based in part on the seizure of small quantities of dynamite near the dam, and the subsequent discovery of a copy of the alert on the Mexican side of the border.

Capt. Francisco Garcia of the Roma Police Department said there was no way to know what the drug traffickers were capable of doing, but bringing down the dam would be tough and require nearly a tractor-trailer full of dynamite.

“As far as blowing it up — making it fall apart completely — it would have to be something like 9-11,” he said. “By the time they'd start to do something, there will be so much law enforcement there it'll be ridiculous.”

Houston Chronicle Staff Writer Dudley Althaus contributed to this report