Since the Virginia Tech shootings last spring, the Federal Bureau of Investigation (FBI) has more than doubled the number of people nationwide who are prohibited from buying guns because of mental health problems.
U.S. officials were quoted by Friday's Washington Post as saying that the FBI's "Mental Defective File" has ballooned from 175,000 names in June to nearly 400,000, primarily because of additions from California.
The names are listed in a database that gun dealers are supposed to check before completing sales.
The surge in names underscores the size of the gap in FBI records that allowed Seung Hui Cho to purchase the handguns he used in April to kill 32 people and himself at the Virginia Tech campus in Blacksburg, Va.
A Virginia state court found Cho to be dangerously mentally ill in 2005 and ordered him to receive outpatient treatment.
But because Cho was not ordered into hospital treatment, the court's order was never provided to the FBI and incorporated in its database.
Two gun dealers checked the list before selling Cho the 9mm Glock 19 and the Walther 22-caliber pistol he used in the shootings.
For nearly four decades, federal law has prohibited gun sales to people judged to be "mentally defective," but enforcement has been haphazard.
A 1995 Supreme Court ruling barred the federal government from forcing states to provide the data, and 18 states -- including Delaware and West Virginia -- provide no mental health-related information to the FBI at all.