The Justice Department is taking steps to require law-enforcement agencies across the nation to report how many people their officers kill — but it will be up to the Trump administration to enforce the new rules.

As my colleague Carl Bialik explained on Thursday, government data on police killings is a mess. Official estimates, which are based on voluntary reports from police departments, miss roughly half of all arrest-related deaths. Amateur data gatherers have tried to fill in the gaps using information from news stories and other unofficial sources; on Thursday, the Bureau of Justice Statistics released its own new estimate, which largely agreed with the amateur efforts, based on a review of hundreds of thousands of reports from the media and other online sources.

That kind of painstaking process wouldn’t be necessary, however, if law-enforcement agencies were better about reporting deaths. In 2014, Congress passed the Death in Custody Reporting Act, which requires departments to report arrest-related deaths and deaths in police custody. On Friday, the Justice Department announced new rules in conjunction with the law and released a report laying out how the new data-collection effort will work.

Starting in July, departments will have to start producing quarterly reports on the number of people who die in custody or during encounters with police. The Justice Department also plans to continue using media reports to make sure local agencies aren’t missing any deaths, although it said that process is too expensive and time consuming to be a “sustainable long-term solution.” And crucially, departments that don’t comply with the new rules risk losing up to 10 percent of the federal funding they receive under the government’s Justice Assistance Grants program.

Those penalties, however, are at the discretion of the attorney general, and past efforts to collect good data on police killings have foundered due to weak enforcement. It isn’t clear how committed Donald Trump’s nominee to lead the Justice Department, Alabama Sen. Jeff Sessions, will be to the new rules.