The news from Fort Hood shocked the nation: American soldiers shot on American soil. Thirteen dead and 38 injured. It was almost too terrible to believe. Almost.

Unfortunately, the Fort Hood rampage was not the first time that our military personnel have been murdered in the United States this year. In June, Abdulhakim Mujahid Muhammad shot and killed an Army private and wounded another soldier at a military recruiting station in Little Rock.

In both cases, the loss of these young soldiers was compounded by a disturbing reality: The assailants had been under investigation by the FBI. In the more recent case, it would be easy enough to point fingers at the FBI. Its counterterrorism agents concluded that Maj. Nidal Hasan's communications with the radical cleric Anwar al-Aulaqi — an al-Qaida sympathizer who acted as a "spiritual adviser" to two of the Sept. 11, 2001, hijackers — were for professional reasons.

A full investigation will reveal whether other red flags should have resulted in preventive action, but here is one thing we already know: A federal law supported by Congress interfered with the FBI's ability to find out about Hasan's purchase of a handgun. Knowledge of that purchase might — and should — have triggered great scrutiny. And it could have saved lives.

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 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.

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.

The Bush administration asked Congress to authorize the FBI to block gun sales to terrorist suspects, to no avail. Last week, Attorney General Eric Holder reaffirmed the Obama administration's support for this legislation — for good reason. A Government Accountability Office report published in June found that individuals on the terrorist watch list had purchased guns and explosives from licensed dealers in the United States on 865 occasions over the past five years.

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. If lawmakers fail to close gaps in the background check system and reform the Tiahrt amendments, the next terrorist shooting on American soil will be shocking, but it will not be surprising.