The sheriff said many of the pirates are teens or preteens, and some "barely even know how to use a weapon."
But despite that, he said he has long feared that someone would be killed.
"The one thing I dreaded on Falcon Lake has happened," he said. "The lake is not secure, the border is not secure because the incident that I dreaded the most has in fact happened. We cannot go to Mexico, we cannot recover that body, we cannot conduct an investigation, we have to tell the family we can't do anything about it."
According to ABC Rio Grande, Texas, affiliate KRGV-TV, there have been at least four previous incidents on the lake over the past five months:
April 30: Four heavily armed men boarded two boats near the Old Guerrero area demanding money.
May 6: Two armed men approached a boat and demanded cash.
May 16: Five armed men boarded a boat on the United States side of the lake.
Aug 31: Pirates, using a small boat marked "Game Wardin" tried to stop a Texas fisherman.
State Representative Aaron Pena, a south Texas lawmaker briefed on the pirates earlier this year, has suggested that residents stay off of Falcon Lake altogether.
"I wouldn't do it. When I go out there I have all the protection Texas can provide. But the average fisherman doesn't have that," Pena said.
Pena added that he is certain that the pirates are working in operation with Mexico's violent drug cartels, who wouldn't allow the pirates on the lake otherwise.
Texas public safety officials said that the pirates use AK-47s or AR-15s to intimidate their victims. They also said that they believe they use local fisherman to man the boats to get as close as possible to American vessels.
Texas Gov. Rick Perry said this week's deadly shooting points to the need for further border security.
"It's really become substantially worse in the last 18 months with the drug cartels having almost free rein," Perry said Friday. "This is about our citizens', on both sides of the borders, safety."