Brigadier-General Qassim Moussawi, an Iraq military spokesman, confirmed that "Iraqi security forces arrested a number of security guards from a security company who fired at civilians today in Karrada district".
"The actions of these contractors will ultimately reflect on coalition forces in general"
Kyle, Fremont, US
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He said the security company "is a foreign company". The nationalities of those arrested is yet to be confirmed.
Iraqi soldiers detained them at a checkpoint inside Karrada and they were still with the Iraqi government, Moussawi said.
Moussawi said the guards were in a convoy of four four-wheel-drive vehicles crossing al-Kahramana Square on the edge of Karrada and were driving on the wrong side of the road when a woman crossing the street was shot.
Iraqi security forces took her to hospital but her condition is unknown.
Many Iraqis view the thousands of private security guards working in Iraq as private armies who can act with impunity.
Al Jazeera's Cookson reported an Iraqi man telling him that he was "very happy in my heart" over Monday's arrest of the foreign security guards.
The Blackwater shooting enraged Iraqis, prompting the government to draw up and approve a draft law to end the 2004 decree granting immunity from prosecution.
That law must still be passed by parliament before it comes into effect.
US media reported on Monday that the US justice department had subpoenaed Blackwater guards to testify before a grand jury as part of its probe into the fatal shootings.
Last week, the New York Times reported that the FBI had found in its initial investigation that at least 14 Iraqis were killed without justification in the September 16 shooting in a Baghdad square.
Blackwater, which guards US embassy staff in Baghdad and visiting dignitaries, says its guards acted lawfully and were responding to a hostile threat to a convoy.
The incident prompted a number of investigations, and the Pentagon and US state department have since agreed to tighten rules governing private security contractors.
Iraq says there are more than 180 mainly US and European security firms in the country, with estimates of the number of private security guards put at between 25,000 and 48,000.