Meghan Hoyer

USA TODAY

The FBI%27s Supplemental Homicide Data includes information on more than 13%2C000 murders in the U.S. in 2012

When 26 teachers, students and administrators were shot to death at Sandy Hook Elementary School, it made national news for weeks. But there was one place 2012's largest mass killing was never mentioned: the FBI database that tracks all U.S. homicides.

And that isn't the only major case missing. The 12 people who were killed in an Aurora, Colo., movie watching the premier of a Batman movie aren't listed either, raising questions about the accuracy and usefulness of the federal data.

Information on more than 13,000 murders from 2012 is in the FBI's supplemental homicide data, which was released earlier this year. The data provides researchers and policymakers the age, race and sex of victims and offenders, the types of weapons used, the circumstances behind a killing and the relationship between killer and victim – information used to craft weapons laws, define the severity of gang killings and research issues such as domestic violence.

A USA TODAY investigation into mass killings has found that the FBI's homicide data over the past decade has only a 57% accuracy rate when it comes to recording the killings of more than four people in a single event. That takes into account cases that aren't there, such as Sandy Hook, and cases that are recorded as mass killings but shouldn't be, such as the July 2012 fatal shooting of a 14-year-old Cleveland girl at a birthday party, erroneously entered as a slaying with four teen-age victims.

The records are voluntarily submitted by police agencies, and FBI officials say the Connecticut State Police and Aurora police departments initially provided the information on the year's two largest killing incidents – only to request that it be deleted.

In Aurora, Sgt. Chris Amsler says his department provides data to the Colorado Bureau of Investigations monthly. The FBI database contains information on 18 other homicides in Aurora in 2012.

"We checked our records and found that all data related to the theater shooting was submitted," he said, adding that investigators were still trying to figure out why the incident was later deleted from FBI records.

Connecticut's homicide count is correct, but the FBI's detailed supplementary material includes only the shooting of Adam Lanza's mother at her home in December 2012, just before Lanza went to the elementary school. Lt. Paul Vance says his department submitted a six-page report on the Newtown school victims to the FBI but later identified a mistake. Updated data was provided too late to be reflected in the database, Vance says, but the information should be added soon.

Missing information in the homicide data isn't unusual. The entire state of Florida, for instance, does not submit data to the FBI. And for many years, Nebraska and Washington, D.C., didn't either. But the 57% accuracy rate is based on dozens of cases between 2006 and 2012 that were not reported to the FBI, which USA TODAY found through other records. It also includes erroneous FBI data based on coding errors. Nationally, there are about two dozen mass killings every year.

Criminologist James Alan Fox said his research has found that roughly 90% of homicides are captured through the data, although many more cases are missing pieces of information, such as suspect and relationship details. He has devised a statistical method that accounts for both missing cases and missing information, to give researchers the ability to look at the full scope of the problem. Mass killings are rare among homicides, and missing such data is especially troublesome in such a small subset.

"We can still look at trends and recognize that they are not infallible but they are pretty good," says Fox, a Northeastern University professor and a member of the USA TODAY board of contributors.

Still, Fox says, the loss of major cases was a concern. "I'd certainly hope somebody would ask why" the mass killings weren't included, he says.

The FBI's data system is notoriously old and can keep data only on up to 11 victims per incident – meaning other large-scale mass killings, such as the Virginia Tech shooting in 2007, had to be broken up over several records. The agency hopes to have a new computer system running by the end of the year, FBI spokesman Stephen G. Fischer Jr. said. That system will keep detailed homicide data in nearly real time, eliminating the current 15-month lag in releasing details of crimes to researchers and policy makers.

Contributing: Paul Overberg