It wasn’t easy assembling the data for this report.

No one agency or office compiles a comprehensive list of instances when police use deadly force, so CommonWealth gathered the information from the state’s district attorneys, the State Police, state and federal court records, and the state Department of Public Health, which tracks all shooting injuries and deaths that involve some sort of treatment at a hospital or clinic.

Each source had deficiencies in its data. District attorneys from at least four counties, for example, failed to list at least one case their offices had handled. The State Police data, covering all of the state except Boston, Worcester, and Springfield, where local police handle investigations, failed to include 11 fatal shootings in Plymouth, Bristol, and Norfolk counties. The State Police list, however, included three shootings by state troopers that were not identified by district attorneys.

Boston police, who were involved in 16 shooting deaths since 2002, the most in the state, initially promised to provide information on their cases but never did despite numerous follow-up calls and emails. Officials in the Worcester and Springfield police departments did not return calls or respond to requests for information.

The district attorneys, who decide whether prosecution of a police officer who used deadly force is warranted, responded in very different ways to requests for information on their deliberations. Some provided a list of all shootings in their districts and reports detailing their decisions on whether to prosecute or not. Others simply provided a brief paragraph on each case of deadly force. Some said their decisions were available at a cost after redaction because of legal exemptions and privacy concerns, exemptions not cited by those officials who provided reports.

One assistant district attorney initially declined to provide any information at all, saying the office was under “no obligation to create a record where none exist.” An aide subsequently provided a list of deadly force incidents in the district after the district attorney was told he was the only one in the state who had declined to cooperate.

It’s also tough to put the Massachusetts incidents of police deadly force in context with the rest of the country. Nationally, the FBI gathers data on deadly force incidents involving police, but the information is woefully incomplete because it is only collected from jurisdictions that report statistics to the agency. In 2012, the FBI data indicate there were a total of 410 justifiable homicides nationally by law enforcement officers, defined as the killing of a felon by an officer in the line of duty. The FBI data offer a state-by-state breakdown only for a category that lumps together justified homicides by both law enforcement officials and private citizens. For Massachusetts, that number was two in 2012, well below the eight we uncovered involving just police.

Jack Sullivan is now retired. A veteran of the Boston newspaper scene for nearly three decades. Prior to joining CommonWealth, he was editorial page editor of The Patriot Ledger in Quincy, a part of the...