By Jack Douglas Jr., CBS 11 News

FORT WORTH (CBSDFW.COM) – On a pleasant October day at a Fort Worth gun show, the makings of a murder began.

“That breaks my heart,” said Jim Terrell, the dealer who sold a Romanian-made gun, similar to an AK-47, on Oct. 10, 2010 to a man he described as “friendly … Just kind of casual.”

“There was nothing suspicious about him, in his movements or eye contact with us,” added Terrell, who runs a gun shop in Burleson when he is not working as a North Texas firefighter.

Four months after that purchase in Fort Worth, an assassin would pull the trigger on that same gun in a highway ambush in Mexico, killing Jaime Zapata, an agent with the U.S. Immigration and Customs Enforcement agency.

The murder, on Feb. 15, 2011, was linked to a Mexican drug cartel. And Zapata would be the first U.S. officer to die in the line of duty in Mexico since 1985.

The killing has triggered a firestorm of controversy in Congress, with some powerful lawmakers questioning how the gun –– and many others like it –– were transported across the border, landing in the hands of dangerous drug cartel mobsters.

“This is very troubling,” said U.S. Sen. John Cornyn, a Texas Republican who is one of the leading critics on how the federal government has handled gun-trafficking investigations recently.

Cornyn said he wonders whether the gun from Fort Worth that was used to kill Zapata was among the thousands of weapons lost during an ill-conceived investigation by the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives.

The undercover project, known as “Fast and Furious” and primarily based out of Arizona, was meant to track down deadly drug cartels in Mexico by following U.S. shipments of government-supplied guns.

Cornyn, in an exclusive interview with CBS 11, said he feels certain guns bought in Houston are linked to the “Fast and Furious” operation. And that makes him suspicious of the weapon purchased at the Fort Worth gun show.

“All of this causes me to ask more questions. And the problem is we just don’t seem to be getting the cooperation that we need from Congress, the Attorney General (Eric Holder Jr.) or the Bureau of Alcohol, Tobacco and Firearms … to get to the bottom of this,” Cornyn said.

Federal records obtained by CBS 11 show that only a month after Otilio Osorio purchased the assault weapon in Fort Worth, he and his brother Ranferi delivered a load of 40 guns to a government informant in a store parking lot in Lancaster, east of Dallas.

ATF agents were watching and recording, but let the Osorio brothers drive off after the exchange was complete, the reports say.

Agent Tom Crowley, spokesman for the ATF in North Texas, told CBS 11 there was good reason for letting the gun traffickers drive away, at least initially.

“There was a stop, but it was also a part of another federal law enforcement operation … by taking them down and arresting them at that time would have possibly jeopardized that investigation,” Crowley said.

“None of the tactics used in this investigation were anything similar to what was used in Arizona’s Fast and Furious, including intentionally walking firearms across the border,” he added.

Otilio and Ranferi Osorio have since been arrested and have pleaded guilty to gun crimes in a federal court in Dallas.

Meanwhile, Jim Terrell, the gun dealer, said he feels bad but not responsible, for the path the Romanian assault weapon took after it left his hands.

“We always strive to go above and beyond in our screening process of people who buy firearms,” said Terrell. “We sell them to the good people. We don’t want the bad people to have them –– ever!”