Even before Congress existed, one of the founding fathers, James Madison, saw that it would be important for legislators to double as investigators, in part to keep the president and the executive branch honest. To balance the government's control over the citizens it governed, Madison wrote, Americans had to "in the next place oblige it to control itself" [source: U.S. Senate]. The U.S. Constitution, though, doesn't explicitly spell out Congress's investigative authority. Instead, as court decisions have found, the authority is inferred as part of the "all legislative powers" granted by Article 1, Section 1 of the Constitution [source: U.S. House].
U.S. Supreme Court rulings in 1881 and 1927 give Congress fairly broad powers to investigate just about anything, as long as it's a matter that Congress is entitled to pass legislation or take other action about [source: Curry, et al.]. That makes a Congressional probe different from, say, an FBI investigation, where the object is to determine whether a crime has been committed. But if Congress does find evidence of criminal acts, it can pass that information along to the Justice Department, which has the authority to file charges [source: Hennessey and Murillo].
Most of the investigations that Congress conducts are carried out by standing committees and subcommittees, which typically have responsibility over certain areas, such as banking or intelligence matters, and conduct investigations as part of their regular duties. But both houses also can create special or select committees, temporary investigative bodies that are put together just to dig into a particular burning issue. Select committees require a majority vote in the chamber, which isn't subject to a presidential veto [sources: Senate, Hennessey and Murillo].
A lot of different things can trigger an investigation. Many of them are reactions to big events, such as the 2008 Wall Street meltdown. In other instances, investigative reporting by the news media may lead members of Congress to conduct a probe. In 2015, for example, stories about the death of a Marine Corps veteran who'd been prescribed multiple drugs while under Veterans Administration care, led to a Senate investigation, which the following year concluded that the VA didn't have adequate internal safeguards to protect patients [source: Slack].