A former Citigroup Inc. employee was the alleged middleman in what authorities believe may be an insider trading scheme, according to people familiar with the matter and disclosures in a U.K. court case that wrapped up Thursday.

The U.K.’s Financial Conduct Authority is investigating whether David Johnson, who left Citi in 2013, collected details about impending deals from a former colleague and passed them to traders, the people say. The Securities and Exchange Commission is investigating the allegations, the people say....