The FAA investigation grew out of allegations by a Delta pilot that the airline retaliated against her for raising safety concerns. The Associated Press obtained a copy of an FAA letter sent to the pilot's attorney detailing the investigation. The FAA declined to comment on the probe.

Trump's nominee, Stephen Dickson, is under growing criticism from Senate Democrats over his initial failure to disclose his involvement in the case of the whistle-blowing pilot, who was grounded a few weeks after she raised safety issues to Dickson and other Delta executives.

Dickson authorized grounding the pilot for a psychiatric evaluation. Outside doctors later cleared her, and she has since returned to flying at Delta.

Dickson testified before the Senate Commerce Committee in May, and the committee is scheduled to vote on his nomination Wednesday. The FAA has been without a permanent administrator since January 2018.

Republicans hold the majority on the committee and in the full Senate, and Dickson had initially looked like a cinch to be confirmed. Dickson's failure to disclose his role in the whistleblower's complaint, however, has delayed and raised uncertainty about his fate and emboldened Democrats.

On Friday, the highest-ranking Democrat on the committee, Maria Cantwell of Washington, announced she will vote against Dickson.

"Information brought to our committee in recent weeks calls into question the safety culture that existed under Mr. Dickson that allowed a safety whistleblower to be retaliated against," Cantwell said in a statement. "The nominee's lack of candor about the issue was also troubling."

Another Democrat on the committee, Richard Blumenthal of Connecticut, has said Dickson's failure to disclose the whistleblower case on a committee questionnaire was "deeply concerning and potentially disqualifying."

White House spokesman Judson Deere responded Friday that Trump picked Dickson based on his experience overseeing flight operations at Delta.

"The White House has complete confidence in his nomination and expects him to be confirmed," Deere said.

Administration officials say they consider the pilot's allegations to be a matter for Delta, not Dickson. They say he filled out all the paperwork accurately and has been open with the committee.

The panel's chairman, Mississippi Republican Roger Wicker, told Politico before Cantwell's announcement that he would push the nomination ahead even if Democrats oppose Dickson.

"We're determined to place the right man in the right office, and I think he's the right man," Wicker said.

The Associated Press left multiple email and telephone messages seeking comment from Dickson.

His nomination is coming to a vote as the FAA faces questions about its oversight of Boeing Co. and approval of the company's best-selling plane. Two Boeing 737 Max jets crashed within five months, killing 346 people, and investigations are looking at the role played by new flight-control software.

The case that Cantwell and Blumenthal mentioned involves Delta pilot Karlene Petitt. She gave Dickson and another top Delta executive a report in early 2016 about safety issues, including pilots being forced to fly when fatigued. Petitt also emailed CEO Ed Bastian, who expressed interest in seeing the report and promised to follow up on it.

A few weeks after she met with Dickson and after being interviewed by a Delta investigator, Petitt was referred to a psychiatrist picked by the airline, who determined that she had a bipolar disorder. She was grounded for more than a year until doctors at the Mayo Clinic and a neutral doctor overturned the diagnosis.

Petitt is suing Delta in an administrative proceeding before a Labor Department judge. Her case was first reported by CNN.

The FAA looked into some of Petitt's complaints. While it took no action on several allegations, the FAA told Petitt in late 2016 that it confirmed Delta wasn't counting time that pilots spent commuting by air to assignments toward their maximum workday — a policy that could raise questions about fatigue. A Delta spokeswoman said the policy changed, and FAA took no enforcement action.

In May, the FAA notified Petitt's lawyer that it would open a new investigation based on another complaint by Petitt. The FAA is now examining whether Delta violated standards for carrying out a "safety management system." Such plans are supposed to encourage pilots and other employees to report real or potential safety issues before they cause accidents.

Dickson was Delta's senior vice president of flight operations when Petitt raised her concerns to him. Last October, he sat for a deposition as part of the Labor Department proceeding that stretched nearly seven hours. In March, Trump announced that Dickson was his choice to lead the FAA.

In his questionnaire to the committee, Dickson — a former Air Force pilot who spent 27 years at Delta, starting as a pilot and retiring last year as its top executive over flight operations — said Delta was involved in various legal proceedings but didn't list any. He also didn't mention Petitt's case when answering whether there was anything else the committee should know about him.

In follow-up written answers to senators, Dickson said he consulted with the White House and the Transportation Department, FAA's parent agency, and decided he didn't have to list cases in which he was not named as a defendant.

Dickson also told senators that his role in Petitt's case "was limited to one meeting with the pilot, and providing direction to my leadership team to ensure that the appropriate follow-up actions were completed" and "that the pilot was treated fairly."

During his deposition, Dickson said Petitt "raised some important issues," although he rejected the notion that Delta pressured pilots to fly if they were fatigued. Dickson said he could have vetoed another executive's decision to refer the pilot for a psychiatric examination, but he declined to do so. He said the referral was "a sound course of action based on the diligence that had been performed" and was based on comments she made to Delta officials who interviewed her.

Dickson added that he does not tolerate retaliation against any employee who raises safety issues, and he called employee reporting "the very core of the safety program."

Dickson was responsible for pilot training and scheduling at Delta. He testified that he left it to a lower-ranking executive to investigate Petitt's claims about issues, including whether Delta had an effective safety management program.

Petitt's lawyer, Lee Seham, said the Labor Department trial on the pilot's retaliation claim against Delta "confirmed that under Dickson's watch there was this near total failure to implement a paradigm program of the FAA (which says) this is how we prevent cataclysms in the airline industry."

The safety management system, he said, is the linchpin of the FAA's safety policy because pilots and aircraft mechanics know about problems and should be encouraged to report them. Yet he said Delta, under Dickson's command, didn't put the system in place.

"How can you expect this man to enforce policy when he couldn't even enforce it at Delta?" Seham asked.