Job offers at the federal level are normally tentative, meaning they may be rescinded at any time. These reasons are usually based on unfavorable information gleaned from previous employers, background check information or the results of a drug screening. As long as the revocation does not violate the candidate's civil rights or discriminate against their race, gender or sexual orientation, no specific reason need be provided.
Double-check that the job offer spelled out that the offer was specifically deemed "tentative" or "contingent." This is usually contained in the main body of the offer letter. These terms help ensure that the letter could not be read as a contractual guarantee of employment.
Request that the rescinding document be reviewed and approved by the legal department. There cannot be any implied promise -- legally known as promissory estoppel -- of a job, nor can there be any hint of discrimination or bias tied to the cause of the change of heart.
Ensure all background checks and security screenings are accurately and thoroughly completed to the best of your knowledge. Tentative federal job offers normally come with the caveat that the hiring is contingent on successful passing of these hurdles. If your candidate has unfavorable information on these checks, you may state this as the cause on the rescinding letter.
Review the candidate's health screening information. If the individual cannot reasonably perform the job at hand based on physical or mental deficiencies -- or would pose a risk to others -- you may have cause to rescind an offer while staying in line with parameters set in the Americans with Disabilities Act.
As a representative or official with the federal government, it's unlikely you can be personally named in a labor law dispute.