WASHINGTON - Military contracts in the Iraq theater have cost taxpayers at least $85 billion since the 2003 invasion, and, when it comes to providing security, they might not be any cheaper than using military personnel, the Congressional Budget Office reported yesterday.

The study was released amid increased scrutiny of military contractors, some of whom are being investigated in the shooting deaths of Iraqis and the accidental electrocutions of U.S. troops.

The United States has relied more heavily on contractors in Iraq than in any other war to provide services ranging from food service to guarding diplomats. About 20 percent of funding for operations in Iraq has gone to contractors, the report said.

There are 190,000 contractors in Iraq, or about one per U.S. service member, the report said.

The study does not include monetary figures for this year. If spending for contractors continues apace, $100 billion will have been paid to contractors for operations in Iraq by the end of the year.

Sen. Kent Conrad (D., N.D.), chairman of the Senate Budget Committee, which requested the review, said the Bush administration's extensive reliance on contractors was a risky precedent.

The use of contractors "restricts accountability and oversight, opens the door to corruption and abuse, and, in some instances, may significantly increase the cost to American taxpayers," Conrad said in a statement.

The death of a Green Beret from Pittsburgh, Sgt. Ryan Maseth, who was electrocuted in January while showering in Iraq, prompted a House committee oversight hearing last month into whether contractor KBR Inc. had properly handled the electrical work at bases it maintains.

The military has also said five other deaths were due to improperly installed or maintained electrical devices, according to a report by the House committee.

In another matter, a federal grand jury is investigating whether Blackwater Worldwide guards acted illegally when they opened fire in a busy Baghdad intersection in September. Seventeen Iraqis died, and the shooting strained relations between the United States and Iraq.

Executives from Blackwater, based in Moyock, N.C., have said they plan to scale back their security contracting business and focus on other areas, in large part because of the negative attention after the shooting.

The Congressional Budget Office estimated yesterday that $6 billion to $10 billion had been spent on security work alone, which it said was comparable to the cost of having a U.S. military unit do the work.

It said 25,000 to 30,000 employees of security firms were in Iraq this year.

The use of military contractors dates to the American Revolution. During the Vietnam War, U.S. contractors were targeted by protesters who accused them of profiting from the war.

Since the end of the Cold War, the military has relied more heavily on contractors - Americans and other nationals - as it has reduced its size. In the Iraq theater, contractors have performed duties that would have required more troops.

The personal cost to many of the employees has been great, including kidnappings.

At least 1,200 have died - including four Blackwater employees ambushed in 2004 by insurgents in Fallujah who strung their remains from a bridge.