Leaping headlong into the fray that is the federal probe of public corruption in the city’s contracting practices, San Francisco City Attorney Louise Renne on Tuesday sued a municipal contractor for fraud and making false claims.

Renne accused the contractor, Scott-Norman Mechanical Inc., of lying to city officials to gain minority business certification. The certification enabled the company to win at least $20 million worth of city contracts at San Francisco International Airport, which is undergoing a $1 billion expansion.