Congress has served as an investigative body many times, including during the Warren Commission, the McCarthy era, and more contemporarily steroid use in baseball. Congressional authority to investigate matters is spread throughout many committees and sub-committees. The authority to act as a judicial body is not directly guaranteed by the Constitution, but is rather an implied power that was first used in the St. Clair Disaster in 1792.
The Warren Commission was designed to look into the assassination of President John Kennedy. It consisted of Chief Justice Earl Warren and several members of Congress, as well as others, looking into the affair. The Commission determined Lee Harvey Oswald acted alone, which has led to many controversies since the decision.
A more contemporary investigation by Congress is the investigation into steroid use in baseball. This is an example of Congress using its authority to regulate a non-government activity, much like it did with investigations of Communist activity in Hollywood in the 1940s and 50s.
Currently, Congress is looking into several scandals involving former Secretary of State Hillary Clinton. The Benghazi investigation into the death of a US Ambassador is being conducted. Congress is also working with the Department of Justice to determine if she violated federal laws with a private email server during her tenure. While the investigations are ongoing, it should be noted Congress has a history of partisan political action, and no conclusion can be drawn from the mere existence of an investigation.