The Commission shall investigate a complaint alleging a discriminatory housing practice and determine, based on the facts, whether probable cause exists to believe that a discriminatory housing practice has occurred or is about to occur. Unless it is impracticable to do so, the Commission shall complete the investigation and make the determination required under paragraph (1) of this subsection within 100 days after the filing of the complaint. If the Commission is unable to complete the investigation and make the determination required under paragraph (1) of this subsection within 100 days after the filing of the complaint, the Commission shall notify the complainant and the respondent in writing and include the reasons for the delay. A person that is not named as a respondent in a complaint, but that is identified as a respondent during an investigation, may be joined as an additional or substitute respondent after written notice in accordance with § 20–1021(e) of this subtitle. In addition to meeting the requirements of § 20–1021(e) of this subtitle, the notice shall explain the basis for the Commission’s belief that the person to whom the notice is addressed is properly joined as a respondent.