Republican Sen. Charles Grassley of Iowa said he had received information that appears to partially corroborate the claim received by members of the Senate Judiciary Committee about the guns.
"Members of the Judiciary Committee have received numerous allegations that the ATF sanctioned the sale of hundreds of assault weapons to suspected straw buyers, who then allegedly transported these weapons throughout the Southwest border area and into Mexico," reads a letter sent Monday from Grassley to Kenneth Melson, acting director of the federal Bureau of Alcohol, Tobacco, Firearms and Explosives.
The letter does not elaborate on the role possible of federal agents in the sale of the guns, and it could not be determined if the purchases were part of a sting operation.
ATF spokesman Tom Mangan declined to comment. Grassley spokeswoman Beth Pellett Levine declined to comment on the senator's letter to Melson.
Border Patrol Agent Brian A. Terry was waiting with other agents on Dec. 14 in a canyon near the Arizona border city of Nogales when a shootout with bandits erupted.
Terry was part of an elite squad similar to a police SWAT team that was sent to the canyon 13 miles north of the border known for robberies, drug smuggling and violence.
No other agents were injured in the attack. The six suspects were being held on felony immigration charges and haven't been charged in Terry's death. All have made court appearances, but authorities declined to release their identities and hometowns.
FBI spokesman Manuel Johnson declined to comment on the investigation into Terry's death. The agency declined an earlier Freedom of Information Act request by The Associated Press to release reports and other records of the investigation.
Grassley said in a letter Thursday that a buyer purchased three assault rifles with cash more than a year ago in Glendale, Ariz., and two of those guns were used in the shootout in Arizona.
"These extremely serious allegations were accompanied by detailed documentation which appears to lend credibility to the claims and partially corroborates them," Grassley wrote.
In the follow-up letter to Melson, Grassley said an ATF manager in Phoenix questioned an agent who answered questions posed by Grassley staffers about the agency's initiative to reduce the flow of firearms to Mexico.
The manager accused the agent of misconduct for his contacts with the judiciary committee, Grassley said.
"This is exactly the wrong sort of reaction for the ATF," Grassley wrote. "Rather than focusing on retaliating against whistleblowers, the ATF's sole focus should be on finding and disclosing the truth as soon as possible."