From http://abcnews.go.com/US/wireStory?id=1087205
"At the Superdome, we have a report that one shot was fired at a Chinook helicopter," Schneider said, adding that the Chinook is "an extremely large aircraft."
Laura Brown, a Federal Aviation Administration spokeswoman in Washington, said she had no such report.
"We're controlling every single aircraft in that airspace and none of them reported being fired on," she said, adding that the FAA was in contact with the military as well as civilian aircraft.
I’m not quoting this to prove that the helicopter shootings didn’t happen, but rather to show that it’s possible that they did not happen, and that the rumor of the shots was enough to ground the rescue fleets regardless of the actual facts.