ATTORNEY GENERAL Eric H. Holder Jr. on Thursday announced that the Justice Department will conduct a formal civil rights investigation of the embattled police department in Ferguson, Mo. Federal officials will also begin an “intensive review” of the St. Louis County Police Department , which was involved in controversial riot control operations in Ferguson. That’s all good — but it cannot mark the outer boundaries of the national reaction to the unrest.

Mr. Holder cited anecdotal accounts of past misconduct to justify the Justice Department’s widening role in Ferguson, a town rent by mistrust of police before and after the shooting of unarmed teenager Michael Brown. Investigators will look for a “pattern or practice” of repeated civil rights violations, including possible racial profiling and excessive use of force. Critics accused the Justice Department of acting on political motivations and pre-judging the guilt of Ferguson police.

Any investigation must be and, we trust will be, fair. Part of that process is calming tensions so that facts and reason have a chance to prevail over anger and distortion. Since the Brown shooting, the Justice Department has pacified where local police have inflamed, using its credibility as an independent force to promise an inquiry outside of local battle lines. Mr. Holder’s personal visit to Ferguson last month also helped to reduce the unrest.

Civil rights investigations of local police forces often lead to agreements among the federal government, local authorities and community groups to reform police training and policies, a process that by itself can begin to improve local relations. Indications are that Ferguson, a majority-African American town with a nearly all-white police force, needed this kind of effort well before Mr. Brown died on the street. All of these reasons help explain why both police departments under review are welcoming the federal scrutiny.

Yet the situation in Ferguson has raised broader issues than the training and conduct of police in the St. Louis area, and it demands a broader response. Top on the list must be informing the national debate on police use of force with reliable figures. Congress in 1994 told the Justice Department to collect and publish national numbers on the excessive use of force, but federal officials have never managed to do it. Those numbers that are available are uninformative for various reasons. Congress should fix this problem. Along the way, lawmakers could review what sort of military hardware the Pentagon is transferring to local cops and why.

Last but far from least, police departments should quickly equip officers to wear live cameras on their uniforms in every reasonable circumstance. Congress should also do more to encourage adoption of the technology. Various studies and anecdotal reports have found that, though far from a panacea, this technology has a variety of potential benefits. Complaints of police mistreatment, for example, have gone down after cops began wearing the devices. We don’t know if Michael Brown would be alive today if Ferguson police had to wear cameras. But having them in place almost certainly would have helped.