By Sue Pleming

WASHINGTON, Sept 27 (Reuters) - U.S. security contractor Blackwater, under investigation in the shooting deaths of 11 Iraqis this month, came under criticism again on Thursday over an ambush in 2004 in which four of its staff were killed.

A report by the U.S. House of Representatives Committee on Oversight and Government Reform detailed a chaotic run-up to the March 31, 2004 incident in Fallujah when four Blackwater employees escorting a convoy were killed and their charred bodies hung from a bridge.

Gruesome photos of their mutilated bodies were published in major newspapers and the incident turned American public opinion sharply against the war. A few days later, the U.S. military launched a major offensive in Fallujah.

The report by Democrat Rep. Henry Waxman's committee criticized Blackwater for ignoring "multiple warnings" of the dangers of going into Fallujah and of sending staff into the "hottest zone in Iraq in unarmored, underpowered vehicles."

Blackwater rejected the conclusions of the report, calling it a "one-sided version of this tragic incident."

"What the report fails to acknowledge is that the terrorists determined what happened that fateful day in 2004," the company said in a statement.

"The terrorists were intent on killing Americans and desecrating their bodies. Documents that the committee has in its possession point out that the Blackwater team was betrayed and directed into a well-planned ambush," it added.

But Waxman said the committee's research showed that leading up to the incident, Blackwater was an "unprepared and disorderly organization" operating in a hostile environment.

"Mistake apparently compounded mistake," he said.

The report was based on documents and interviews with 18 individuals with knowledge of the incident, including Blackwater's Baghdad operations manager and project director and seven other staff from the company.

A day before the incident, the report said Blackwater's operations manager in Baghdad sent an e-mail to company headquarters in North Carolina complaining about a lack of equipment, including armored vehicles, ammunition and weapons.

"I need new vehicles, I need COMS, I need ammo, I need Glocks and M4s ... I've requested hard cars from the beginning," said the e-mail.

Blackwater provides security for the U.S. embassy in Baghdad and at the time of the Fallujah incident, the company was taking over operations from a British security company.

It is currently subject to a joint U.S.-Iraqi investigation into an incident on Sept. 16 when 11 Iraqis were killed after Blackwater guards opened fire while escorting a convoy through Baghdad. The State Department's own diplomatic security is also looking into the incident.

"Something went tragically wrong on September 16 and we are taking steps to address the matter," Deputy Secretary of State John Negroponte, a former ambassador to Iraq, said in a statement to Congress.

Negroponte said from January this year until Sept. 18, Blackwater conducted 1,873 missions protecting diplomats or visitors outside of the Green Zone in Baghdad and on 56 of those missions weapons were fired.

He provided no details of the incidents but said they were reviewed to ensure procedures were followed.

"I personally was grateful for the presence of my Blackwater security detail, largely comprised of ex-Special Forces and other military, when I served as ambassador to Iraq. Their alert and controlled posture kept me safe, to get my job done," said Negroponte.

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