Private security contractors in Iraq could face prosecution in the US if they commit crimes, under a bill passed by the US House of Representatives.
Blackwater provides security to US diplomatic staff in Baghdad
The vote comes amid investigations into the role of the private security firm, Blackwater in the fatal shooting of 11 Iraqi civilians in Baghdad last month.
Senate Democratic leaders said they planned to vote on a similar bill soon.
Meanwhile, the state department said it had handed over its inquiry into the incident to the FBI.
The shift occurred in preparation for the possibility that the case could be later referred to the US justice department or Iraqi authorities for further action, the White House said.
An FBI spokesman said criminal charges were possible against Blackwater staff if its inquiry agreed with the Iraqi government's findings.
Blackwater's chief insists his firm's personnel acted in self-defence
Iraq has accused Blackwater's employees of opening fire on civilians at al-Nisoor square in Baghdad without provocation, a charge the firm denies.
Blackwater's founder Erik Prince has insisted his firm's personnel acted in self-defence after insurgents attacked the US diplomatic convoy they were protecting.
The Democrat-controlled House of Representatives voted 389 to 30 in favour of a new law that would make all private contractors working for the US government subject to US laws.
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
Currently the legal status of private contractors working independently of the US military in war zones is unclear and legally untested.
Private firms working for the Department of Defence are subject to existing US legislation, but those private firms such as Blackwater working for the state department are not.
Democratic Representative David Price, who sponsored the bill, said it was hard to believe such a "gaping hole" existed in US law.
"Unlike the military, there is no clear chain of command for contractors, little in the way of standards for training and vetting personnel and no legal accountability for misconduct," he told the House of Representatives on Wednesday.
The state department itself has acknowledged that there may be gaps in existing legislation and has promised to look into the matter.
Blackwater has earned more than $1bn (£490m) from US government contracts since 2001.
The state department paid the company more than $832m (£408m) for security work between 2004 and 2006.