Texas Gov. Greg Abbott has vowed to defy the Biden administration after a federal agency instructed him to remove what was built on an island local officials said has become a hot spot for Mexican drug cartels. 

In a letter Monday to President Biden and Vice President Harris, Abbott said the U.S. Section of the International Boundary and Water Commission (U.S. IBWC) told the Texas General Land Office (GLO) to restore Fronton Island to "pre-construction conditions." 

Fronton Island sits on the border between the U.S. and Mexico on the Rio Grande. The 170-acre island has seen an assortment of cartel activity, including gunfights and illegal immigration.

"For years, the cartels were ‘running rampant’ in Fronton Island’s ‘thick vegetation’ and ‘bullet-pocked structures’ along the river to stage illegal entries, surveil state and federal law enforcement, stash weapons, plant explosives, evade apprehension, and engage in open warfare against rival cartels and against state and federal officers," Abbott wrote. 

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This map shows the location of Fronton Island on the Rio Grande. (Google Maps/Texas General Land Office)

"On one occasion recounted in the citations below, authorities found 'six assault rifles, a grenade launcher, a rocket launcher, 20 ammunition magazines for various-sized weapons and three packages of what appeared to be C-4 plastic explosives' hidden on the Island," he added. 

"On another, Texas soldiers found an improvised explosive device 'buried amid a stash of weapons and semi-automatic rifle ammunition.'"

Texas Gov. Greg Abbott has vowed to defy the Biden administration over an island on the Rio Grande. (Mega/Getty Images)

The Texas Department of Public Safety and Texas Rangers were recently given permission to patrol and police the island by local authorities. 

"Law enforcement has found some ammunition, suspicious devices on it. And we just feel like with the border crisis, it was really important to ensure that Texas law enforcement have access to these islands so we can help secure our border," Texas Land Commissioner Dawn Buckingham previously told Fox News Digital. 

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This map shows Fronton Island on the Rio Grade between Texas and Mexico. (Gov. Greg Abbott)

In 2021, Abbott launched Operation Lone Star, an aggressive effort to combat the flow of illegal immigrants and drugs into Texas from Mexico. Texas National Guard soldiers and law enforcement have also set up razor wire fencing and other barriers along the border and have floated buoys on the Rio Grande to make it more difficult to cross. 

In addition, the GLO has built two sediment bridges, placed concertina wire and "cleared vegetation" on the island, Abbott said. The U.S. IBWC, the American partner in the International Boundary and Water Commission, is a joint body formed by the U.S. and Mexican governments to regulate the southern border. 

The Biden administration said the island is part of federal land. However, Abbott said the island is owned by Texas. 

"Your open-border policies have allowed an invasion at the southern border and incentivized criminal activity that threatens the lives of Texas law enforcement, soldiers, and citizens," he said. "Yet, in the wake of a crisis that it helped create, the federal government has refused to enforce federal laws – even in dangerous areas like Fronton Island."

Fox News Digital has reached out to Abbott's office, the White House and the IBWC.

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The effort has resulted in clashes between Abbott and the Biden administration. Texas secured the island in October, saying it had been used by Mexican drug cartels as a base. 

"For years, Fronton Island was overrun with violence. Texas secured it against the cartels," Abbott wrote on X. "The Biden-Harris Admin now demands that Fronton Island return to its prior condition. Today, I told Biden & Harris that Texas will not comply."