The prevents them from asking before extending an offer, but it seems they could (and should) have checked after.[0]

> However, an employer may ask about criminal conviction(s) after extending a conditional offer of employment (the employer can never ask about arrests or criminal acusations that aren't pending). An employer who properly asks about a criminal conviction can only withdraw the offer or take adverse action against the applicant for a legitimate business reason that is reasonable under the six factors* listed in the Act.

One of the six factors is "Fitness or ability of the person to perform one or more job duties or responsibilities given the offense"[1], which they probably could have invoked after asking (though they never checked or didn't check thoroughly enough, so I guess it's moot).

[0]https://ohr.dc.gov/page/returning-citizens-and-employment

[1]https://ohr.dc.gov/sites/default/files/dc/sites/ohr/publicat...