(CNN) — Federal agents can’t account for more than 1,400 guns after a widely criticized operation aimed at tracing the flow of weapons to Mexican drug gangs, sources with knowledge of the investigation tell CNN.

Of 2,020 guns involved in the Bureau of Alcohol, Tobacco, Firearms, and Explosives probe dubbed “Operation Fast and Furious,” 363 have been recovered in the United States and 227 have been recovered in Mexico. That leaves 1,430 guns unaccounted for, the sources said.

The ATF operation was intended to build cases against Mexican drug cartels by allowing firearms to go from the United States into Mexico. The hope was by tracing the guns in Mexico, agents would be able to determine the structure of various cartels and then bring them down.

The problem was that once the guns were allowed to “walk,” there was no way to recover them until they turned up at crime scenes. The operation has been widely criticized in Congress, with the chairman of a House committee that investigated the issue calling it “felony stupid.”

Rene Jaquez, the ATF’s former attache in Mexico City, told CNN the operation never should have happened.

“Guns traditionally are just not allowed to leave the undercover operation for fear that it will enter into the criminal element and then be subsequently used in a crime at a later date,” Jaquez said.