Last December, The Trace received one of the first big batches of records for the project that would eventually become “Missing Pieces.” In response to a request for public records, the Baltimore Police Department sent us a spreadsheet containing more than 14,700 records — including serial numbers — of guns that had been recovered in the Maryland city over the previous seven years.

The response also contained a 2,673-page document with information about guns that had been reported stolen in the city. It was a PDF, and the stolen guns’ serial numbers were scattered between messy reports of other weapons. In order to standardize our data, we would need to isolate the serial numbers of the stolen weapons we were looking for and manually convert the PDF to a spreadsheet.

It was the first major indication that our reporting objective — understanding the relationship between stolen guns and crime guns in America — would be anything but easy to complete.

Over the next 11 months, with the valuable assistance of partners at more than a dozen NBC TV stations, we acquired huge numbers of records — more than 800,000 — from 1,054 law enforcement agencies in 36 states and Washington, D.C. Reporters and editors at The Trace and NBC obtained the data by filing public records requests for data in the property and evidence-management systems of major cities and counties around the United States, as well as in smaller jurisdictions in and around NBC markets. Some of the records were clean and ready to import into our spreadsheet. Many others were like those from Baltimore and required painstaking attention.

In some cases, The Trace and NBC had to fight to obtain records from agencies that sought to withhold gun serial numbers under exemptions in state public-records laws. The Trace won an administrative appeal for serial numbers against the District of Columbia. The Chicago Police Department released its serial numbers after the law firm Loevy & Loevy filed a lawsuit on behalf of The Trace.

The database contains nearly complete stolen-gun records for the states of California and Florida, both of which have centralized collections of gun-theft data. Data for all other locations was obtained from individual law enforcement agencies. Wherever possible, we have standardized column names and values in accordance with National Crime Information Center gun-data codes.

Our main goal in amassing this data was to better understand what happens to guns after they are reported stolen. We were able to match more than 23,000 stolen guns to weapons later recovered by law enforcement. We, along with our partners in major cities across the country, used these records to identify homicides, robberies, sexual assaults, and other violent crimes committed with stolen guns. Now we are releasing all of the data in the hope that other newsrooms nationwide — and anyone else who is interested — will use it for further investigations.

Our complete data, including matches, is available for download in the CSV format below. We are also including a dictionary explaining the nature of the data, the efforts we took to clean and standardize it, and a detailed explanation of how matches are identified and interpreted. Finally, we have published a separate listing of the agencies from which we acquired data, including the number of records provided and the years covered.

If you would like to access the raw data provided by individual agencies, please contact us at [email protected]. And if you publish a story that uses the data we collected, please let us know!

— Daniel Nass and Brian Freskos