The DOT's Office of the Inspector General will conduct an audit of the FAA's oversight of United Airlines' maintenance program.
The audit will begin later this month.
The audit follows a string of safety-related incidents at United that drew national headlines in March.
In its announcement of the probe, Nelda Smith, DOT's assistant inspector general for aviation audits, said that the department has conducted a series of similar audits over the past five years, scrutinizing Allegiant, Southwest and American. An audit of regional carrier SkyWest is also underway.
"Recent safety events with United Airlines -- such as flight diversions that can be traced to mechanical problems -- serve to remind us that FAA oversight of maintenance programs is paramount," Smith wrote.
During the audit, the DOT will evaluate the FAA's actions to address maintenance noncompliance and violations at United.
In late March, the FAA announced that it would increase oversight of United, including dispatching more officials to review work processes and manuals and to examine United facilities.