something goes into interstate commerce or someone or something is in violation of federal law (or the Constitution). The point is that the Justice Department is investigating what appears to be a national pattern of police abuse, where this case in Missouri is just one of many such potential cases.
You have to understand the role of the federal government versus state or local governments. And you also have to understand the mandate of the Civil Rights Division of the Justice Department, which was established by one of the earlier Civil Rights laws -
About the Division
The Civil Rights Division of the Department of Justice, created in 1957 by the enactment of the Civil Rights Act of 1957, works to uphold the civil and constitutional rights of all Americans, particularly some of the most vulnerable members of our society. The Division enforces federal statutes prohibiting discrimination on the basis of race, color, sex, disability, religion, familial status and national origin.
Since its establishment, the Division has grown dramatically in both size and scope, and has played a role in many of the nation's pivotal civil rights battles. Division attorneys prosecuted the defendants accused of murdering three civil rights workers in Mississippi in 1964, and were involved in the investigations of the assassinations of Dr. Martin Luther King, Jr., and Medgar Evers. The Division enforces a wide array of laws that protect the civil rights of all individuals.
http://www.justice.gov/crt/about/