DALLAS (AP) — Southwest Airlines continues to fly airplanes with safety concerns while federal officials do a poor job overseeing the airline, a government watchdog said Tuesday.

The airline has flown more than 150,000 flights on 88 jets it bought on the used-plane market and which had unconfirmed maintenance histories, the Transportation Department's inspector general said in a report. That put more than 17 million passengers at risk, according to the report.

In 2017, FAA inspectors began finding "potentially serious gaps" in Southwest's process for verifying the condition of the planes, including major repairs that weren't documented and maintenance records that didn't meet FAA standards.

Meeting U.S. standards normally takes up to four weeks per plane, but people hired by Southwest approved 71 of the planes on the same day, the inspector general said.

Southwest said 80 of the planes have been inspected and returned to flying, and the last eight are undergoing maintenance.

The FAA gave the airline until this summer to bring the planes in compliance with federal rules because it accepted Southwest's argument that the issues were low safety risks, the inspector general said. The watchdog office added that FAA has not given its inspectors enough guidance on reviewing risk assessments and evaluating an airline's safety culture.

"As a result, FAA cannot provide assurance that the carrier operates at the highest degree of safety in the public's interest, as required by law," the inspector general said. That is so even though "FAA representatives — ranging from senior executives to local inspectors — expressed concerns about the safety culture at Southwest Airlines."

Southwest said it has taken steps to address the report's key findings, and it "adamantly" disagreed with the critique of its safety culture.

"Southwest maintains a culture of compliance, recognizing the safety of our operation as the most important thing we do," airline spokeswoman Brandy King said in a statement. The airline works "to improve each and every day, any implication that we would tolerate a relaxing of standards is absolutely unfounded."

The report is another setback for the FAA, which is being investigated by Congress for its approval of the Boeing 737 Max, the plane that was grounded after two crashes killed 346 people. Critics have said the agency is too cozy with airlines and aircraft manufacturers.

Sen. Roger Wicker, R-Miss., chairman of the Senate Commerce Committee, said the report highlighted "very concerning lapses in FAA's safety oversight." He said his committee is investigating many of the same issues.

The inspector general said the FAA accepted Southwest's justification that any lapses at the carrier "were low safety risks." It said the agency has failed to strike a balance between collaborating with industry and managing safety safety risks.

"FAA cannot provide assurance that the carrier operates at the highest degree of safety in the public's interest, as required by law," the report said.

The FAA agreed with all 11 of the inspector general's recommendations to improve oversight of Southwest, including new training for inspectors who monitor the nation's fourth-biggest airline.

The inspector general began investigating FAA's oversight of how Southwest handles risk after an engine explosion caused a passenger's death in April 2018.

The review found a number of problems. In addition to insufficient maintenance records on used planes, for nearly two years Southwest frequently failed to give pilots correct information about the weight and balance of loads on their planes, which the inspector general called an important safety lapse.

Last month, the FAA proposed a $3.9 million fine for improper weight calculations on more than 21,500 flights. Southwest can fight the penalty.

Southwest has said it has improved its system for calculating weight and balance of cargo.

The report also said FAA did not evaluate Southwest's risk assessment after a hard landing last year during dangerous gusty winds — higher than Southwest pilots are trained for — at Bradley International Airport in Connecticut. Both wings of the Boeing 737 were damaged when they hit the runway during the first of three attempted landings before pilots flew to another airport, where they landed safely.

The incident was first reported last month by The Wall Street Journal.