- Diamond Detective Agency
 - Chicago Heights - Offers investigative and security services for both private and public sectors. - Don Haworth & Associates, Inc.
- Chicago - Midwest private company offering a variety of investigation and surveillance services.
- Harpernet Investigative Services
- Valparaiso - Private detective agency specializing in video surveillance and helping find people.
- Investigative Services Agency
- Chicago - Provides security, corporate and private investigations, and corporate consulting in the Chicagoland area and beyond.
- Investigators of Illinois, Inc.
- Chicago - Full service private detective agency, including security consulting, criminal defence investigations, domestic investigations,and armed bodyguard protection.
- On-Line Screening Services, Inc.
- Chicago - Online pre-employment background verification.
- Robert Arden and Associates
- Northbrook - Provides online employee screening solutions, applicant background checks, and related human resources services.
- TSS Consulting Group
- Gurnee - Private investigation consulting company specializing in PI training, security audits, crisis management, and more.
- Wicklander-Zulawski and Associates
- Downers Grove - Provides interview training, background investigations, undercover agents, secret shoppers, and more to the public and private sectors.
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