United Airlines may be fined nearly $600,000 after the company's drug testing process for flight crews failed to give them an equal chance of being selected for random drug and alcohol testing, the Federal Aviation Administration alleged Friday.

Thirteen United Airlines workers in key safety-sensitive positions were not properly tested for drugs, the FAA alleged

Federal official proposed the fine on Friday, saying the company broke rules for random drug and alcohol testing.

United did not perform pre-employment drug tests and receive verified negative test results before transferring 13 individuals to safety-sensitive positions, the FAA alleges.

United also failed to use a scientifically valid method to ensure that each member of the company's flight crews has an equal chance of being selected for random drug and alcohol testing each time a selection is made, the FAA also alleges.

Drug testing is both a critical and a required safety measure that all operators must follow, said FAA Administrator Randy Babbitt.

United has 30 days to respond to the enforcement letter.

Safety is United's top priority. We are reviewing the letter and will cooperate fully with the FAA to resolve their concerns, United Airlines said in statement, according to CNN.