The US security firm Blackwater is at the very centre of the controversy surrounding the "outsourcing" of war, where private contractors are taking on tasks usually carried out by government soldiers.
Private security firms take on some of the most dangerous tasks
Based at a vast ranch complex in North Carolina and calling itself "the most comprehensive professional military... company in the world", the firm is under investigation after a Baghdad gunfight on 16 September in which 17 civilians were killed.
In 2004, the firm hit the headlines when the burnt and mutilated bodies of its contractors were hung from a bridge across the river Euphrates in the Iraqi city of Falluja, to the cheers of an angry mob.
Four Blackwater men died during that ambush but, despite the deaths, the company's contractors remain among the most numerous in Iraq.
Critics say such contractors, often elite soldiers recently retired from military special operations units, are little more than mercenaries, awarded lucrative packages to fight on demand in lawless areas.
And, according to human rights campaigners, their uncertain legal status in Iraq - straddling international law, US regulations and Iraqi legislation - enables them to act with virtual impunity.
Blackwater provides protection for US officials in Iraq, guarding convoys such as the state department one attacked on Sunday.
Such firms are often the last link in a chain of contracts and sub-contracts awarded by government agencies and bodies.
Blackwater is one of the main private providers of security within Iraq, along with US firms DynCorp International and Triple Canopy - as well as the UK's Aegis.
And according to a recent report by the US Congressional Research Service (CSA), the firm has the largest number of contracts for high-risk "personal protective services" awarded to a US firm in the country.
The firm's founder is a 38-year-old former US elite navy commando
A law drawn up in 2004 by the Coalition Provisional Authority (CPA) - the now-defunct interim body set up by the US-led coalition in the wake of the fall of Saddam Hussein - grants such firms immunity from prosecution under Iraqi law.
Blackwater was one of the first private security firms to work on the ground in Iraq following the US-led takeover, securing the contract to provide security for CPA head Paul Bremer.
This was among the first of the many lucrative government contracts that are reported to make up 90% of the company's revenues.
Laundry to live rounds
Although it is difficult to know how many staff the company employs in the country, the CSA cites reports of some 182,000 people being employed under US government contracts.
The bulk of these are Iraqis, their tasks ranging from the mundane, such as laundry services, to the downright dangerous - such as guarding convoys and infrastructure sites likely to be targeted by insurgents.
BLACKWATER USA FACTS
Founded in 1997 by a former US Navy Seal
Headquarters in North Carolina
One of at least 28 private security companies in Iraq
Employs 744 US citizens, 231 third-country nationals, and 12 Iraqis to protect US state department in Iraq
Provided protection for former CPA head Paul Bremer
Four employees killed in Falluja in March 2004
Blackwater's base in North Carolina is reported to span several thousand acres and to include vast firing ranges, tactical exercise areas and an armoury.
Its founder, 38-year-old Erik Prince, is a former member of the US Navy's elite special operation force, the Navy Seals.
Originally concentrating on providing gun and firearms training for US government agencies, after the attacks of 11 September 2001, the company evolved and now defines itself as "not simply a 'private security company'".
"We are a professional military, law enforcement, security, peacekeeping, and stability operations firm who provides turnkey solutions," its website reads.
The company mission statement explains it can help draw up "national and global security plans" and "train, equip and deploy public safety and military professionals".
Mr Prince has shown himself to be as able an entrepreneur as a soldier, ever quick to spot new business opportunities.
Blackwater provided security in New Orleans after the city was pounded by Hurricane Katrina.
And, according to a book on the firm written by Jeremy Scahill, in the aftermath of the 1999 Columbine school shootings in Colorado, the company built a fake school on the grounds where police teams could practise tracking down high-school killers to the sounds of screaming students.
Yet despite the concerns about the privatisation of war and the lack of accountability of such fighters, some analysts say that without the use of security firms, sustaining ongoing operations in Afghanistan and Iraq would simply be impossible.