THE EFFICACY OF GUN DATA…. We’ve learned in recent weeks that Nidal Hasan’s communications with a radical cleric had come to the attention of the FBI, which had begun investigating the Army psychiatrist accused of the Fort Hood massacre. Federal officials did not, however, know about Hasan’s purchase of a handgun — a move that would likely have brought greater scrutiny before the shootings.

New York City Mayor Michael Bloomberg (I) and former New Jersey Gov. Thomas Kean (R) write this morning that the FBI couldn’t have known about Hasan’s firearms purchase because of a shift in the law, approved several years ago. The curb on gun data, they argue, can and should be fixed.

During the Clinton administration, the FBI had access to records of gun background checks for up to 180 days. But in 2003, Congress began requiring that the records be destroyed within 24 hours. This requirement, one of the many restrictions on gun data sponsored by Rep. Todd Tiahrt (R-Kan.), meant that Hasan’s investigators were blocked from searching records to determine whether he or other terrorist suspects had purchased guns. When Hasan walked out of Guns Galore in Killeen, Tex., the FBI had only 24 hours to recognize and flag the record — and then it was gone, forever.

As former FBI agent Brad Garrett has said, “The piece of information about the gun could have been critical. One of the problems is that the law sometimes restricts you in what you can do.”

The Tiahrt amendments passed by Congress interfere with preserving, sharing and investigating data on gun purchases by terrorist suspects. If that weren’t bad enough, Congress has also failed to close a gap in federal law that prevents the FBI from blocking a sale to an individual under investigation for terrorist activity.

To put this in a slightly larger perspective, if the FBI is investigating someone who may have terrorist ties, that person will be put on a no-fly list. That same person, however, is free to purchase firearms, and the FBI will likely not know. In other words, those suspected of terrorist activity can’t buy a plane ticket, but they can buy a semi-automatic.

The fatal lesson we learned on Sept. 11 was that, if we are going to protect innocent Americans from terrorists, we must break down the walls standing between federal agencies and effective investigative practices. The attack at Fort Hood was a tragic reminder that such walls still exist. Until Congress shows the political courage to tear them down, there will be more catastrophic breaches of national security and more tragic loss of life.

The Bush administration sought a change in the law, but Congress, listening to the gun lobby, ignored the request. The Obama administration wants the same change — Attorney General Eric Holder reminded lawmakers about this last week — though there’s a limited political appetite for closing the existing gun-data gap.

Here’s hoping the Fort Hood tragedy changes the equation.

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.