Sources tell ABC News that in August 2009, Maj. Nidal Malik Hasan walked into the Guns Galore gun store in Killeen, Texas, and legally purchased the FN Herstal tactical pistol that authorities believe was used to massacre soldiers at Fort Hood.
An FBI background check under the National Instant Background Check System was done when Hasan purchased the pistol -- but that information was never shared with the Joint Terrorism Task Force in Washington, which was aware that Hasan had repeatedly contacted a radical imam suspected of having ties to al Qaeda.
The FBI's Joint Terrorism Task Force ran down intelligence leads relating to Hasan late last year but closed the inquiry sometime in early 2009.
"The piece of information about the gun could have been critical," said former FBI Special Agent Brad Garrett. "One of the problems is that the law sometimes restricts you in what you can do."
As investigators continue to examine whether there were missed signals in the Fort Hood shooting massacre, some individuals are frustrated that there are still gaps in information-sharing, especially when it comes to looking at federal gun laws.
The Fort Hood shooting followed a June incident in Little Rock, Ark., where police say Abdulhakim Mujahid Muhammad opened fire at an Army recruiting station, killing a soldier. Muhammad was under FBI investigation for possible ties to terror and travel to Yemen.
A senior law enforcement official tells ABC News that Muhammad purchased a gun, in the weeks before the shooting, at a department store. An FBI background check was done -- but the FBI counterterrorism investigators working Muhammad's case were apparently unaware.
Senior law enforcement officials say the Brady Law forbids them from widely sharing information about legal gun purchases.
"We need to be smarter about sharing information," said former 9/11 commissioner Richard Ben-Veniste. "It's very disturbing to see…that the FBI is precluded from sharing information."
Current federal law does not prohibit people on the terrorist watch lists from purchasing guns unless there is a prohibiting factor, such as being a fugitive, having a felony conviction or charge, renouncing U.S. citizenship or having been determined as mentally impaired.
Earlier this year, Sen. Frank R. Lautenberg, D-N.J., introduced legislation which would prevent known or suspected terror suspects from purchasing firearms.
"If someone on any terrorist watch list tries to buy a weapon, law enforcement must be informed – period," Lautenberg said in a statement Wednesday when contacted by ABC News. "If some people are being blocked from flying on an airplane, then we should certainly know when they are buying an assault weapon."