The Federal Aviation Administration allowed Southwest Airlines to put millions of passengers at risk by letting the airline operate planes that did not meet U.S. aviation standards and by failing to provide its own inspectors with the training needed to ensure the highest degree of safety, according to a report released Tuesday by the Department of Transportation’s inspector general.

The strongly worded report called the FAA’s oversight of Southwest’s safety management system “ineffective” and said the agency improperly relied on Southwest’s own conclusion that repeated problems represented a “low risk, rather than requiring the airline to comply with its regulatory requirements.”

The inspector general said the “FAA cannot provide assurances that the carrier operates at the highest degree of safety in the public’s interest as required by law.”

Matthew Hampton, the assistant inspector general for aviation audits, said his office received a hotline complaint about FAA oversight in early 2018 that warned of “inaccurate information being provided to pilots prior to flight,” among other issues. Then, that April, an engine failed on a Southwest flight, killing Jennifer Riordan, an Albuquerque mother of two. That combination prompted the broader audit, according to Hampton.

The report found that Southwest “operates its fleet of aircraft with frequent weight and balance errors.” Such incorrect information was “frequently” communicated to pilots, which can “greatly affect” a plane’s safety and performance, the report said.

The report also found that Southwest “operates previously owned airplanes in unknown states of airworthiness.”

Southwest said it has made progress on those issues but disputed that it had any problem with its own “safety culture.”

The inspector general issued 11 recommendations to the FAA, including retraining its inspectors and managers; guaranteeing that Southwest complies with regulatory requirements that 88 used aircraft meet U.S. standards; and making sure that safety management systems airlines put in place, which were supposed to improve transparency and safety, are not used “as a substitute for regulatory compliance.”

In a written response to the inspector general Jan. 30, the FAA said it concurred with the recommendations and would implement all but one by Sept. 30, some significantly earlier.

The final recommendation is that the FAA put in place guidance for ensuring that “risk assessments” provided by airlines are comprehensive and get to the root causes of problems and that those problems actually get fixed. The FAA said it would complete that recommendation by Sept. 30, 2021.

The FAA’s overreliance on industry-provided risk assessments and failure to dig deeply into many of those assessments is a broader concern raised by several outside experts and reviews following the crashes of two Boeing 737 Max jets that killed 346 people. In the case of the Max, the FAA accepted Boeing findings that underestimated risks associated with a flawed automated feature and overestimated pilots’ ability to react if the system failed.

Regarding Southwest, the FAA said in its response that the agency’s certificate management office handling the airline “did not perform in accordance with existing guidance” when it allowed 88 used aircraft to fly using the company’s own process for checking them. That process “lacked a comprehensive conformity inspection,” the report said.

As a result, Southwest operated aircraft with “unknown airworthiness conditions” on more than 150,000 flights, placing as many as 17.2 million passengers at risk, the report said.

The FAA said Southwest has now undertaken a far-reaching assessment of those aircraft, identifying at least 88 cases of repairs that “did not conform,” as well as scores of other less serious shortcomings. The agency said Southwest has reported that all repairs considered “non-conforming” have been replaced.

The FAA also agreed that Southwest “at times” did not follow guidance regarding accurate weight and balance information for pilots, a key issue raised by the inspector general.

“While weight and balance control is critical for the safety of flight operations, the issues at [Southwest] rarely involved a baggage miscount that significantly affected aircraft loading,” the FAA said in its response. Still, it added, “these types of errors remain of concern.”

The FAA said it had appointed a new leadership team at its office managing Southwest “to help remedy systemic concerns with the internal and external relationships” there, as well as “deficiencies” in the work and culture of the office.

The agency said FAA leadership “took or oversaw various actions” once it learned of the issues identified by the inspector general. But an FAA spokesman declined to answer questions about how the agency’s leadership had allowed such problems to arise in the first place or to identify any root causes of the oversight failures identified in the inspector general’s report.

“Our complete response is at the back of the report,” spokesman Lynn Lunsford said.

Even as the report faulted FAA officials for not holding the airline accountable, it also offered a sharp rebuke of Southwest, which has built a loyal following among travelers for its folksy approach to flying.

Unlike its rivals, it doesn’t charge fees for the first two bags, nor does it charge fees to travelers who want to make changes to their itineraries. The approach has enabled the Texas-based carrier to hold its own against mainline carriers and the new breed of ultralow-cost airlines that have entered the market.

But inspectors interviewed for the report raised concerns about Southwest, saying the airline’s safety culture consists of using “diversion, distraction and power” to get what the company wants.

Other FAA officials said the airline bypassed the local oversight office and went directly to FAA headquarters when there was a disagreement.

“They’ve always done that — just more blatant now,” said one FAA official. Another said that the airline’s attitude toward the FAA appeared to take the form of: “I’ll respond to you when I damn well please.”

“We adamantly disagree with unsubstantiated references to Southwest’s safety culture,” the carrier said in a statement. “Southwest maintains a culture of compliance recognizing the Safety of our operation as the most important thing we do. The success of our business depends, in and of itself, on the Safety of our operation, and while we work to improve each and every day, any implication that we would tolerate a relaxing of our standards is absolutely unfounded.”

But 28 of 46 FAA oversight staff interviewed by the inspector general shared concerns about the culture at Southwest, the report said. “Southwest’s management is very skilled in what they have to do. If it costs money, they won’t do it,” an official told investigators.

FAA employees indicated they were at times hamstrung in doing their jobs.

The officials said they couldn’t consider the airline’s safety culture when it made a request to start flying to Hawaii — something the investigators said required “even higher attention to safety” — because the airline met the necessary technical requirements.

The inspector general found that FAA staff wrongly relied on Southwest’s risk assessments to allow it to continue not complying with safety rules. And despite the long-running problem with how the weights and balances of planes were being tracked, the investigators said the FAA’s Southwest oversight office didn’t try to determine the underlying cause of the issue. Instead, they said it was up to the airline to do that.

“These statements are contradictory to FAA guidance,” the investigators wrote.

The safety of used airplanes was another key point of contention.

Typically, it takes three to four weeks to verify the safety of a previously foreign-owned aircraft, according to the report. But in 71 of 88 cases, FAA designees — Southwest employees who are supposed to operate in the agency’s interest — signed off the same day that the plane was inspected at a repair station.

The investigators also concluded that FAA inspectors approved changes to Southwest’s training program “without considering whether the carrier evaluated the potential safety impacts of those changes.” One FAA official called the changes “the biggest change to pilot training in decades.”

While the FAA’s reliance in recent years on a more cooperative “compliance program,” rather than more traditional enforcement “offers a new strategy for working with carriers to address safety risks, the Agency’s oversight must remain robust to ensure carriers identify the root cause of violations and implement effective corrective actions,” the report said.

“Given the significant unresolved safety concerns that FAA has identified at Southwest Airlines, it is clear that the Agency is not yet effectively navigating the balance between industry collaboration and managing safety risk at the carrier,” it said.

“The IG report highlights very concerning lapses in FAA’s safety oversight,” said Sen. Roger Wicker (R-Miss.), chairman of the Committee on Commerce, Science and Transportation, which has been investigating many of the same issues at Southwest that were identified in the inspector general’s report, in part based on information it had received from whistleblowers. “A carrier’s safety culture should be overseen with the highest standards to ensure the traveling public is safe.”

Southwest said that since the inspector general’s office concluded its work in the fall, the airline has made “significant progress” on addressing the issues raised in the report. The airline said it has “enhanced the integrity” of its weight and balance program. It added that the 88 aircraft purchased from foreign carriers have either “completed a comprehensive physical inspection” or are currently being inspected.