Because they are by their nature secret, price-fixing, bid-rigging and similar conspiracies are difficult to detect and prove. Law enforcement officials rely on complaints and information from government procurement officers and business customers, as well as competitors. A large percentage of all government investigations results from complaints received from consumers or people in business.
On the federal level, you can contact the US Department of Justice, Antitrust Division, or the Federal Trade Commission, both of which have headquarters in Washington, DC. In the District government, you can contact the Office of the Attorney General (OAG). Each agency can handle a wide range of complaints; complaints of mainly local significance are more likely to be of interest only to OAG.