Once a Congressional committee begins to investigate something, it has a lot of power. Ever since an 1827 probe in which the House empowered a committee "to send for persons and papers" related to tariff legislation, both the House and Senate have claimed the right to summon anyone inside or outside the U.S. government to testify. Some witnesses may testify voluntarily, but those who are issued subpoenas don't have any choice. Congress passed an 1857 law allowing them to be prosecuted for contempt of Congress if they don't comply. And they have to tell the truth, or face perjury charges [source: U.S. Senate].
But witness testimony in hearings is just the most visible part of the investigation process. Congressional probes can issue subpoenas for documents. Committee staff members — who do much of the digging that Congress members get to take credit for — conduct depositions, in which they take sworn testimony from witnesses in private settings. That arrangement allows them to preview what people might say in an open hearing, and to verify any bombshell assertions before they become public.
It's not hard to figure out that the power to compel people to testify to Congress easily could be misused. To guard against such abuses, the Supreme Court in 1957 ruled that witnesses have to be informed of the precise subject of the investigation, and can't be forced to talk about anything else [source: Curry, et al.] Witnesses also can claim a Fifth Amendment privilege against self-incrimination, and show up for the hearing but refuse to testify, as Col. Oliver North did special House Select Committee to investigate the Iran-Contra Affair [source: Hennessey and Murillo].
In that case, Congressional committees have another tool of last resort. They can decide by a two-thirds vote to grant immunity from prosecution, and then get a federal court to issue an order compelling the witness to testify. But granting immunity can be a risky proposition from a political standpoint. If a witness admits to really heinous crimes and can't be prosecuted for them, the public may unleash some of its outrage on politicians who made that possible [source: Hennessey and Murillo].