The number of people who are barred from buying guns because of mental health problems has more than doubled since the Virginia Tech shootings last April.
The increase follows stepped-up reporting to a federal database used to screen the backgrounds of potential gun-buyers.
Attorney General Michael Mukasey was to announce the increase at an afternoon speech in Park City, Utah. The number of people identified in the database as having mental problems grew from 174,863 three months after the April 16 Virginia Tech shootings to 393,957 this month.
Virginia Tech student Seung-Hui Cho killed 32 people and himself in the deadliest campus shooting in U.S. history. He bought two guns - a Glock 9mm at a Virginia store and a .22-caliber pistol over the Internet - despite a special justice's order in 2005 to get outpatient treatment for being a danger to himself. There has been no indication that Cho ever received treatment.
Had Cho's court order been submitted to the federal database, he probably would have been unable to buy the guns.
"Instant background checks are essential to keeping guns out of the wrong hands, while still protecting the privacy of our citizens," Mukasey said in comments prepared for delivery to the National Association of Attorneys General.
"But as we learned in the tragedy at Virginia Tech, the checks must be accurate and complete to be effective," Mukasey said. "We're making progress, and I hope that even more states will submit this information so that the national instant background check system can be maximally effective."
People are included in the federal database only after courts or other legal authorities have found them to have mental health problems, Justice Department officials said. Currently, 28 states submit names to the mental health database, and the federal government is powerless to force the other 22 to follow suit.
Private mental health records, including diagnosis documents from hospitals or insurance companies, are not accessed or submitted to the database. Justice Department officials said more than 5 million people are identified in the database, which is maintained by the FBI and tracks illegal immigrants and people with criminal histories.