Federal investigators said Thursday that the region’s airports authority is afflicted by a “culture of favoritism” that led to widespread nepotism, mismanagement and questionable contracting practices at an agency overseeing the $6 billion Dulles Rail project, one of the largest public works projects in the country.

The final report from the Department of Transportation’s Office of Inspector General shows that Metropolitan Washington Airports Authority senior staff picked favorites among contractors, including one firm that got a contract even though it was charging 234 percent more than some other bidders. Authority staff took lavish gifts from vendors and skirted personnel rules to get family and friends jobs, inspectors found during a yearlong investigation.