The officer told USA TODAY that four bodies were found aboard the helicopter. The cause of the crash was not immediately known, the officer said.
The Associated Press, however, reported that five Americans were killed in the crash, citing an unnamed U.S. official in Baghdad.
Both the USA TODAY and AP sources spoke on condition of anonymity because they were not authorized to speak about the incident.
Witnesses in the Fadhil neighborhood reported seeing the helicopter go down after gunmen on the ground opened fire and were believed to have shot the pilot or co-pilot or both. Accounts varied, but all were consistent that at least one person operating the aircraft had been shot and badly hurt before the crash.
The helicopter was believed to have been flying escort above a VIP convoy on the ground as it headed away from the heavily fortified Green Zone to an undisclosed destination.
A spokeswoman for Blackwater USA, which is based in far northeastern North Carolina, declined to comment Tuesday. "We really don't have any information for you yet," said spokeswoman Anne Tyrrell. The company provides security for State Department officials in Iraq, trains military units from around the world, and works for corporate clients.
The Blackwater aircraft was at least the 14th helicopter to go down since the war began in March 2003. The worst incident occurred Jan. 26, 2005, when a transport helicopter crashed in a sandstorm in western Iraq, killing 30 Marines and a sailor.
According to insurance claims on file at the Department of Labor, 770 civilian contractors have been killed in Iraq since the war began in March 2003, through Dec. 31, 2006. Additionally, 7,761 civilian contractors have been injured in the same time period, according to claims on file.
Katy Helvenston, mother of Scott Helvenston, a Blackwater employee who died in March 2004, said Tuesday's crash "just breaks my heart."
"I'm so sick of these kids dying," she said.