From a contracting notice published by Social Security:
The Social Security Administration (SSA) intends to issue a sole source contract with cost reimbursement task orders ... to the National Academy of Sciences (NAS) for a period of 5 years beginning on or around September 30, 2018. The NAS will set up Committees of medical, vocational, and other experts having expertise in fields such as, internal medicine, pediatrics, oncology, otolaryngology, ophthalmology, endocrinology, physical medicine and rehabilitation, occupational therapy, physical therapy, orthopedic surgery, industrial and organizational psychology, psychiatry, developmental-behavioral pediatrics, neurology, cardiology, vocational rehabilitation, health care case management, social sciences, education and health care and workplace economics. The Standing Committee will continue work necessary to maintain an essential capability for theoretical analyses of research, relevant evidence and clinical practices in physical medicine, psychiatry, and rehabilitation medicine.
... The size standard is $20.5M....The National Academy of Sciences is not a governmental agency. It's a private non-profit that largely functions as a beltway bandit seeking out this sort of contract.