Air Traffic Organization chief Hank Krakowski resigned Thursday as the Federal Aviation Administration deals with a growing controversy over air traffic controllers falling asleep on the job.

FAA Administrator Randy Babbitt issued a statement that Krakowski, who had overseen the U.S. traffic system since 2007, submitted his resignation Thursday morning and that he accepted it. FAA's chief counsel, David Grizzle, will take his place until a permanent successor can be found.

"Over the last few weeks we have seen examples of unprofessional conduct on the part of a few individuals that have rightly caused the traveling public to question our ability to ensure their safety," Babbitt said in a statement. "This conduct must stop immediately. I am committed to maintaining the highest level of public confidence and that begins with strong leadership."

Five incidents have been reported recently of air traffic controllers falling asleep on the job. The latest involved a sleeping air traffic controller discovered early Wednesday in Nevada. The controller was out of communications for 16 minutes while a medical plane was landing.

No one was injured when the flight carrying at least three people landed at Reno-Tahoe International Airport. The controller has been suspended while the FAA investigates his lack of response to repeated contacts from the plane and airport staff.

An investigation by the FAA uncovered a second incident of an air traffic controller sleeping on the job in February during the midnight shift at McGhee Tyson Airport in Knoxville, Tenn.

The agency also said a controller in Seattle had been suspended for falling asleep during a morning shift on Monday. The controller already faced punishment for falling asleep on two other occasions during an evening shift in January, the FAA said.

In addition, two controllers in Lubbock, Texas, were suspended after two failed handoffs two weeks ago.

Former FAA chief spokesman Scott Brenner called the resignation a "political killing" that was "truly unfortunate."

Brenner told Fox News he was surprised that Krakowski was forced to resign because he had been "making tremendous progress in moving our aviation system into the 21st century."

He added that controllers are part of a strong and effective union that had tied Krakowski's hands on things that he had wanted to do.

"He's got so much on his plate and had done so well and to take a handful of instances ... where guys had fallen asleep or dozed off and to ruin his career and rid him of the agency I think is incredibly short-sighted," he said.

But the incidents had riled U.S. officials and lawmakers.

"I am totally outraged by these incidents. This is absolutely unacceptable," Transportation Secretary Ray LaHood said in a statement Wednesday after the latest incident. "The American public trusts us to run a safe system. Safety is our No. 1 priority and I am committed to working 24/7 until these problems are corrected."

"Air traffic controllers are responsible for making sure aircraft safely reach their destinations. We absolutely cannot and will not tolerate sleeping on the job," Babbitt said in a statement.

Senate Majority Leader Harry Reid said on the Senate floor that he shared LaHood's and Babbitt's outrage.

"This shouldn't happen in Nevada. It shouldn't happen anywhere in the country," he said. "It shouldn't happen to any airplane. And it certainly shouldn't happen to an air ambulance."

Sen. John Rockefeller, chairman of the Senate Committee on Commerce, Science and Transportation, said he told the Babbitt in a phone call that he's "sick of this."

"We can't have an aviation system where some of the people responsible for safety are literally asleep at the switch. This has to stop," he said in a statement. "The agency needs to do whatever it takes to keep air traffic controllers from sleeping on the job or not treating their responsibilities with the highest level of seriousness and attention."