Possible Clues:

• 20-Across, e.g., informally
• Classic Bogart role, in slang
• Sleuth, in old crime fiction
• Investigator regularly observed in the act
• P.I., in old slang
• Columbo, famously (Abbr.)
• Investigator doesn't finish college
• Private Eye shortly gets half of 27 down
• Investigator, note, heading for court
• Detective, slangily