The federal agency that oversees the National Weather Service says warning sirens and notifications went out well ahead of the devastating Joplin tornado. But it says residents didn't respond quickly enough to the sirens warning of the impending twister.

"There is an ambiguity regarding the seriousness of the threat," said Richard Wagenmaker, the National Oceanic and Atmospheric Administration's service assessment team leader, in a telephone briefing for reporters. "In other words, people ask, 'What does the siren mean?  How seriously should I take it?'  And, as a result, most people did not immediately go to shelter."

The NOAA report released Tuesday (9/20), about communications before and during the May 22 tornado that killed 162 people in the southwest Missouri city, says the National Weather Service was well-prepared and "performed in an exemplary manner." The report also says combined efforts from the weather service, emergency management and the public "saved many lives."

But the report also says "the vast majority of Joplin residents" didn't respond to the first siren because of an apparent widespread disregard for tornado sirens. Nearly 100 people in Joplin were interviewed, including storm victims and local media. A number of them suggested there are too many watches and warnings about storms and people become desensitized. One woman's suggestion caught the agency's attention.

"She thought their should be a regular warning and accompanying siren or alert, and then another level of tornado warning in a different tone, or a different siren or alert for when it would be really bad," Wagenmaker said.

Click here to read the entire assessment.

(The Associated Press contributed to this report.)