A Federal Aviation Administration official wrote a memo last month saying that Southwest Airlines Co. should ground 49 of its airliners because repairs were performed that didn't meet legal standards.

There is “a high likelihood of a violation of a regulation, order or standard” of the FAA, and the U.S. regulator needs to take immediate action to revoke the certification of the planes, said H. Clayton Foushee, the agency’s director of the Office of Audit and Evaluation. The Oct. 24 letter was released Monday by the U.S. Senate.

The FAA wrote to Southwest days later ordering the airline to speed up inspections of Boeing Co.’s 737 NG planes, which were previously owned by foreign carriers. But it has stopped short of requiring that the planes be grounded. The agency said in a statement that a risk assessment had concluded the airline had taken appropriate measures.

The dispute within the agency over the status of the planes comes as the Transportation Department’s inspector general completes an audit of FAA’s oversight of airline maintenance and as lawmakers assess whistle-blower claims that the agency hasn’t been aggressive enough.

The FAA is also under fire for its approval of the 737 Max, the newest model of Boeing’s single-aisle workhorse, which has been grounded since March after two fatal crashes.

Sen. Roger Wicker, the Mississippi Republican who is chairman of the Commerce, Science, and Transportation Committee, wrote to the FAA on Oct. 30 saying he finds the situation “troubling.”

Southwest chief executive officer Gary Kelly told employees in a weekly message on Monday that the company had found that “a small number of repairs on a few of these aircraft” had been performed but not properly classified by the previous owners. In some cases, language differences or repair criteria were to blame, Kelly said.

“Our continuous assessments of the ongoing inspections has revealed nothing to warrant the expedited timeline,” Kelly said of the FAA action, but Southwest remains on track to have them completed by the end of January.

In its Oct. 29 letter to Southwest, the FAA said it was considering additional action, including grounding the planes, if the carrier couldn’t justify the safety of its fleet.

Concerns over whether the planes — of a group of 88 obtained since 2013 from carriers in countries including Kenya, Colombia, Russia, and Turkey — meet safety requirements have been dogging Southwest for more than a year. The airline grounded 34 of the planes last year during the busy Thanksgiving travel period over issues with repair records.

As of Oct. 4, Southwest had performed thorough inspections of 39 of the 88 planes, Foushee said in his memo. Of those, 24 were found to have “significant airworthiness issues,” he said. The issues included previously unknown repairs and major maintenance found to have been done improperly, he said.

Numerous FAA inspectors in the office overseeing Southwest have told Foushee’s office that “they are no longer comfortable with the current corrective action plan,” he said in a footnote of the memo.

The Wall Street Journal reported earlier on whether the Southwest jets meet FAA standards.