Heavily staffed and well resourced, the dozens of committees set up by the two houses of the US Congress have few limits on what they can do.
Republican Senator Norm Coleman has led the UN probe
Of them all, the Permanent Subcommittee on Investigations, a unit of the Senate's Committee on Homeland Security and Government Affairs, has the freest hand.
It has the power to examine both government efficiency and the behaviour of corporations and individuals.
It can recommend legal changes "to protect public interests against the occurrence of improper practices or activities by labour or management groups".
It can investigate national security and energy matters. And it can look at all kinds of crime, organised or otherwise.
In practice, committee members have interpreted their mandate as widely as possible - usually in line with their own interests.
Carl Levin has focused on following the money
For the chairman, Republican Norm Coleman, the priority most recently has been to strike out at what he perceives as corruption at the United Nations - and to call for the departure of Secretary General Kofi Annan.
The committee's investigation into the UN's oil-for-food programme, and Iraq's alleged allocations of oil to politicians in the UK, France, Russia and elsewhere, has brought it into the spotlight.
For the leading Democrat on the committee, Carl Levin, the main focus has been follow-the-money investigations, including wide-ranging inquiries into money laundering, Enron and most recently tax evasion.
Among other ongoing projects are investigations into nuclear terrorism and port safety; the provision of credit to low earners; the abuse of first-class air tickets at the defence department; music piracy, and the Sars virus.
The committee is now a firmly established arm of government but it has a stormy history.
It was set up in 1948 to look at waste and impropriety in government, and promptly took up the issue of corrupt lobbyists - implicating several staff of President Harry Truman's White House.
Soon afterwards in 1953, Senator Joseph McCarthy became chairman - using his position to bolster his campaign against alleged Communists. But his use of the committee soon tarnished his crusade, and he was censured by the Senate in 1954.
A year later, Robert Kennedy - who became attorney general when his brother John F Kennedy entered the White House - was made the committee's chief counsel, in a period which focused on labour unions' abuse of power.
After 1960, it moved onto investigating the Mafia, leading to the Racketeering Influenced and Corrupt Organisations (Rico) Act - still the cornerstone of the US government's anti-mob activities.
The late 1960s saw investigations into government contracting, while the 1970s added energy shortages to the agenda in the wake of "oil shocks" as well as national security issues.
In the 1980s and the 1990s, issues examined included investment fraud, child pornography, drug policy and computer security.