Washington ? Hired to protect U.S. diplomats, private security companies like Blackwater can own their own helicopters, buy their own body armor and set their own rules of engagement. Their guards don’t answer to the U.S. military or, as recent cases suggest, anyone else.

This reality has aroused Congress, where the House passed a bill Thursday that would make all private contractors working in Iraq and other combat zones subject to prosecution by U.S. courts. It was Congress’ first major response to a deadly shooting last month involving Blackwater employees.

Democrats called the 389-30 vote an indictment of the shooting incident there that left at least 13 Iraqis dead. Senate Democratic leaders said they planned to follow suit with similar legislation and send a bill to President Bush as soon as possible.

“There is simply no excuse for the de facto legal immunity for tens of thousands of individuals working in countries” on behalf of the United States, said Rep. Sheila Jackson-Lee, D-Texas.

The FBI arrived in Baghdad on Thursday to investigate the Sept. 16 shooting, although administration officials acknowledge they are unsure whether U.S. courts would have jurisdiction in the case or others like it.

In a separate incident, a drunken Blackwater employee left a Christmas Eve party in Baghdad and fatally shot the guard of one of Iraq’s vice presidents. That contractor was fired, fined and returned home to the United States, but no charges have been filed.

The current law, called the Military Extraterritorial Jurisdiction Act, covers personnel supporting the mission of Defense Department operations overseas. But because Blackwater’s primary mission is to protect State Department officials, defense lawyers would likely argue that the law doesn’t apply.

At the same time, all U.S. contractors are immune from prosecution by Iraqi courts under an order signed in 2004 by U.S.-appointed occupation governor L. Paul Bremer.

The White House and congressional Republicans said they support the intent of the bill, but thought it was drafted poorly.