Private investigators

Foot surveillance, photography, and telegraph (Pinkerton agency model)

The detective tools of the Pinkerton era were field craft and documentation: foot surveillance, disguise, the cultivation of informants, and the systematic filing of case records and mugshots. Pinkerton's agency maintained what contemporaries described as the world's largest collection of criminal photographs and identification records by the early 1870s, predating the FBI's own files by decades. Photography (then a relatively new technology) was used from the outset to document suspects and evidence. The telegraph enabled the agency's nationwide network of offices to share information on fugitives in near-real time. The detective's primary instrument remained human observation and human memory, organised through Pinkerton's pioneering case-file system.

Telephone, portable cameras, and audio recording devices

The 1920s marked the first era in which private investigation became accessible to the average American client, not just corporations and railroads. The telephone enabled investigators to conduct background inquiries and verify information without leaving their offices, and to maintain real-time contact with field operatives. Portable 35mm still cameras (the Leica launched in 1925) gave investigators a far more discreet documentation tool than the bulky plate cameras of the Pinkerton era. Divorce litigation was a central market during this period: fault-based divorce laws required documented evidence of adultery, desertion, or cruelty, creating sustained demand for photographic surveillance. Miniature recording devices, developed partly from wartime intelligence techniques, expanded audio capabilities. Harold Lipset, a San Francisco investigator who operated from the 1940s through the 1960s, pioneered miniaturised surveillance equipment and became a national figure in debates about the ethics of electronic eavesdropping.

Video camcorder and sub-rosa surveillance (workers-compensation era)

The portable video camcorder, which became commercially practical in the mid-1970s and cheap enough for widespread field use by the early 1980s, transformed insurance-fraud and workers-compensation surveillance. Where an investigator had previously needed to be in close proximity to take usable photographs, a camcorder with a telephoto lens could record continuous activity from a parked vehicle at a distance, producing evidence that was far more persuasive to juries and insurance adjusters than still photographs. Workers-compensation fraud surveillance ("sub rosa" investigation, from the Latin for "under the rose," meaning covert) grew rapidly after states expanded workers-compensation systems in the 1970s. Insurance companies routinely hired investigators to document claimants who appeared to perform physical activities inconsistent with their claimed injuries. This single work category drove major employment growth in the profession from the 1970s through the 1990s.

Commercial databases, Internet OSINT, and GPS tracking (digital information era)

The 1990s brought two transformations that reshaped the investigator's daily toolkit: the commercialisation of public records through database aggregators (LexisNexis, Accurint, IRB Search, TLO) and the emergence of the Internet as a source of personally identifying information. Where a skip trace in 1985 required physical visits to county courthouses and motor vehicle offices, by 2000 an investigator could retrieve credit-header data, utility records, vehicle registrations, and prior addresses from a laptop in seconds. GPS tracking devices, enabled by civilian access to the US GPS satellite network after the May 2000 removal of Selective Availability, became field tools by the early 2000s. The profession also absorbed two digital specialties: computer forensics (recovering deleted files, email, and metadata from hard drives) and what would come to be called OSINT, open-source intelligence gathering from public web sources, social media, and data-breach exposure.

Social media OSINT, drone surveillance, digital forensics, and AI-assisted research (contemporary era)

The present era is defined by the volume and variety of digital trace evidence available from social media platforms, financial transaction records, and mobile-device location data. Investigators now routinely use social media analysis tools (Paliscope, Social Links, Babel Street) to track subjects' public posts, check-ins, and associations. Drone surveillance, legal for investigative use in most public contexts, adds an aerial dimension to physical surveillance. Digital forensics now encompasses cryptocurrency tracing (recovering assets moved through blockchain transactions) and mobile-device data extraction. AI-assisted tools are beginning to enter the profession: facial recognition software can scan thousands of images across public databases, and language-model tools assist with summarizing large document sets in litigation-support cases. The core tasks that resist algorithmic substitution remain physical surveillance (a drone pilot must make real-time judgment calls), live-witness interviewing (which requires human social intelligence to be effective), and court testimony (which requires a human who can be cross-examined). Mainstream tools as of 2026 include TLO and IRB Search for data queries, Cellebrite or Oxygen Forensics for mobile-device extraction, and DJI drones for aerial surveillance.