In his Friday morning news conference, National Rifle Association chief executive Wayne LaPierre floated the idea of a national registry of the mentally ill as one way to stem gun violence.

"How can we possibly even guess how many, given our nation’s refusal to create an active national database of the mentally ill?" he asked. 

Turns out, many states are ahead of him: 38 states require or authorize the use of certain mental health records for use in a firearm background check, according to the Law Center to Prevent Gun Violence, a San Francisco-based nonprofit that tracks state level gun legislation.

The Gun Control Act of 1968 prohibits gun sales to individuals who have been committed to a mental institution or "adjudicated as a mental defective." 

"The federal law pretty much says that anyone who has been involuntarily committed or anyone who has been determined by a court to be a danger to themselves or others, those two categories are prohibited from possessing firearms," says Lindsay Nichols, an attorney with the Center to Prevent Gun Violence. 

That prohibition stands today but is pretty difficult to enforce, partially due to the issue LaPierre mentioned: Reporting on mental health status is incomplete, with a hodgepodge of state laws that specify what information does, or does not, go into the federal background check system.

The issue came to a head after the Virginia Tech shooting in 2007. Shooter Seung-Hui Cho was, under federal law, ineligible to purchase a firearm after a Virginia judge had declared him to be a danger to himself. At that time, however, Virginia had no law requiring reporting of mental health records to the federal database. Cho's disqualifying diagnosis was never transmitted into the background check database. 

Shortly after that shooting, Congress allocated additional funds for states that shared at least 90 percent of their mental health records with the federal government, which they could put toward their criminal justice programs. 

A report from Mayors Against Illegal Guns found that, between August 2010 and October 2011, mental health records sent to the federal database increased by 35.4 percent.

Thirty states (including Virginia) now either authorize or require reporting mental health records to the National Instant Criminal Background Check System, or NICS. Those are the states in dark red below. Another eight states, in lighter red, authorize or require reporting of mental health records to a separate, state-level database.

Each state law has a bit of a different flavor to it. Alabama, for example, only requires reporting of an involuntarily commitment if there is "evidence that the person has a history of inappropriate use of firearms or poses a threat to use firearms inappropriately." In Hawaii, health-care providers and public health authorities are responsible for providing information in cases where the county police chief requests it. Illinois requires immediate reporting to the state police when a person has been adjudicated as a "mental defective," as defined by federal regulations.

Creating a national database, Nichols says, would be a difficult task to pull off, and not just because of the "nation's refusal" that LaPierre cited.

The federal government does not have the constitutional authority to require state agencies to report data. The most it can do is offer funding — or withhold dollars — in an attempt to entice states to participate, just as they did with the law after the Virginia Tech shooting. 

Many anti-gun advocates push for better mental health reporting, including Mayors Against Illegal Guns. It recommended in a November 2011 report that "Congress should significantly increase both the federal funding available to assist record sharing and the penalties for states that do not comply, and tie them to far more ambitious reporting targets."

Nichols too believes that better state tracking of mental health records, and stronger reporting to the federal government, would be a positive step for gun control.

"State legislators need to pay great attention to this," she says. "They need to have the capacity to manage their own affairs and help enforce these prohibitions."