WHAT IS BLACKWATER?

* Blackwater is one of the biggest security contractors in Iraq. It employs more than 1,000 people there and is responsible for guarding U.S. diplomats and Embassy security.

* Its distinctive small black helicopters hover above Baghdad and its armed vehicles shadow convoys of senior officials through the city's streets.

* Blackwater USA consists of nine business units, ranging from canine to parachute units, maritime security and the manufacture of custom armoured vehicles.

WHERE DOES IT OPERATE?:

* As well as Iraq, it operates in Afghanistan and has also had domestic contracts, such as guarding and policing New Orleans after Hurricane Katrina.

* Based in Moyock, North Carolina, it says it is "the most comprehensive professional military, law enforcement, security, peacekeeping, and stability operations company in the world."

WHO RUNS IT?:

* The secretive company was founded in 1997 by former Navy SEALs Erik Prince and Al Clark.

* Clark has since left but media-shy CEO Prince -- a right-wing billionaire Christian -- has made substantial donations to Republican politicians.

WHO ARE THE SECURITY CONTRACTORS?:

* Salaries -- reportedly as high as $1,000 a day -- attract troops and ex-Special Forces personnel from countries such as Bosnia, the Philippines, Israel and Chile. Tens of thousands have streamed in since the U.S.-led invasion in 2003.

WHAT IS THEIR ROLE IN IRAQ?:

* They have duties once performed by armed forces such as airport and border security. Other contractors cook soldiers' meals, do their laundry and drive truck convoys.

HOW MANY CONTRACTOR CASUALTIES HAVE THERE BEEN?:

* At least 647 contractors were killed between March 1, 2003, and Sept. 30, 2006.

WHAT'S THE WIDER TREND?:

* The "privatizing war" trend has been accelerating steadily since the end of the Cold War, when the United States and former adversaries began cutting back professional armies.

* The contractors' murky status as civilians in war zones has led to concern in the military over how they fit into the chain of command.

CRITICS AT HOME AND ABROAD:

* Critics say hired security are paid mercenaries. Many Iraqis believe they operate outside the law with little accountability either to the Iraqi government or U.S. military.

Sources: Reuters,