WASHINGTON - July 28 - The Coast Guard isn't prepared to fight an offshore oil rig fire and is investigating whether the chaotic spraying of salt water by private boats that rushed to the Deepwater Horizon explosion contributed to its sinking.

Coast Guard officials told the Center for Public Integrity that the service did not have ships in the area equipped to combat the blaze after the BP-operated rig exploded on April 20, leaving the work instead to a half-dozen private boats that answered the initial distress call.

An official maritime investigation led by Coast Guard Capt. Hung M. Nguyen in New Orleans is examining whether the haphazard firefighting efforts - in which salt water was sprayed across the burning platform - overran the ballast system that kept the rig upright, changing its weight distribution, and causing it to list.

Making matters worse, the Coast Guard failed to put an expert in charge of the firefighting effort as required by its own disaster manual and could not directly communicate with the private vessels, according to testimony reviewed by the Center.

The joint investigation by the Coast Guard and the Interior Department is one of 10 formal inquiries since the offshore oil well blew out, killing 11 workers and unleashing millions of gallons of oil into the Gulf of Mexico in the worst spill in U.S. history. The disaster entered its 100th day on Wednesday.   Read and link to the full story of the Coast Guard BP rig firefighting effort.