The Bureau of Alcohol, Tobacco and Firearms -- the agency tasked with keeping U.S. guns from being smuggled to Mexico -- has come under fire for allegedly allowing firearms to cross the border into Mexico.

Last Friday, Sen. Charles Grassley (R-Iowa) sent a letter to the ATF stating that his office had "received numerous allegations that the ATF sanctioned the sale of hundreds of assault weapons to suspected straw purchasers, who then allegedly transported these weapons throughout the Southwest border area and into Mexico."

Grassley asked for ATF officials to meet with his staff to discuss the matter, noting that "there are serious concerns that the ATF may have become careless, if not negligent, in implementing the Gunrunner strategy." Gunrunner is the name of the ATF operation to keep guns from entering Mexico.

On Monday, a concerned Grassley sent a follow-up letter, writing that while ATF had not yet responded to his request for a meeting, one of the whistleblowers that Grassley's office had been dealing with -- a current ATF employee -- was "allegedly accused... of misconduct" by his boss for talking with Senate staffers.

Note: See letters here and here. The letters initially surfaced on the website of gun rights blogger David Codrea. Sen. Grassley's Press Secretary Beth Levine confirmed to FoxNews.com that the letters were genuine.

ATF spokeswoman Janice Kemp referred questions from FoxNews.com to spokesmen Drew Wade and Scot Thomasson, who did not respond to calls or e-mails from FoxNews.com on Tuesday morning.

But a former ATF agent told FoxNews.com on condition of anonymity that ATF Headquarters allowed guns to cross the border, for the reason that ATF wanted to glean more intelligence about who would come to possess the guns and what regions of the country they would end up in.

Additionally, Dick Deguerin, attorney for a Texas gun store named "Carter Country," told Fox26 Houston that the ATF asked the store to sell the guns to even those they thought were going to smuggle them to Mexico -- so that ATF could track where the guns went.

"They were told to go through with sales of three or more assault rifles at the same time... They went through with the sales because the ATF told them to go through with the sales," Deguerin said, adding that the store reported all suspicious sales to the ATF.

Federal investigators are known to use such techniques in drug trafficking investigations.

"Controlled delivery is an investigative technique that allows specific consignments of illegal drugs or other controlled substances to pass through the territory of one or more states with the objective of identifying not only the street dealers, but the individuals controlling the drug trade network," a State Department website notes.

However, the ATF has not admitted to using that technique for gun trafficking investigations. While ATF did not respond to requests for comment from FoxNews.com, Mexican newspaper El Diario El Paso reported last week that ATF spokesman Scot Thomasson told them, "we do not permit the exit of arms to Mexico."

Grassley's first letter to the ATF hinted at the potential problems with allowing firearms to cross borders, writing that two of the weapons that the ATF allegedly sanctioned to be sold to straw purchasers were sold to Mexican gangs and "were then allegedly used in a firefight on December 14, 2010, against Customs and Border Protection agents, killing CBP Agent Brian Terry."

Gun rights bloggers have speculated that the reason for allowing the guns into Mexico was to pad statistics on the number of guns crossing the border -- the suspicion being that a higher number would make the ATF's mission in preventing the guns from crossing seem all the more urgent.

The former ATF agent who spoke with FoxNews.com said that he had no reason to think that padding the statistics was a motivation.

FoxNews.com previously reported that an often-quoted statistic that 90 percent of guns used in Mexico crimes came from the U.S. was, in fact, a myth. Only 17 percent of guns found at Mexican crime scenes have actually been traced to the U.S.

None of the claims about the ATF allowing guns to cross the border have been conclusively proved. But Sen. Grassley has called for the ATF to be more transparent to allow the truth to come out.

"This is exactly the wrong sort of reaction for the ATF," Grassley's second letter reads.

"Rather than focusing on retaliating against whistleblowers, the ATF's sole focus should be on finding and disclosing the truth as soon as possible."