The private investigation field originated in 1850, when Allan Pinkerton, a former police force detective, established the National Detective Agency in Chicago. The agency grew to be the largest private security company in the industry. Railroad robberies were prevalent at the time and railroad companies hired Pinkerton's men to help them enforce laws and capture outlaws. Federal agencies did not exit at the time and local authorities didn't have the manpower to handle crimes out of their jurisdictions, so gangs of train robbers and other criminals could cross state lines and local territories freely.
Pinkerton's private investigation agency helped track down and capture outlaws and also performed other duties that are associated with federal and state law enforcement agencies' purview today, such as guarding interstate railroad and stagecoach shipments, investigating crimes, and providing advice on security measures for banks and other businesses that outlaws frequently targeted.
In the 1900s, federal and local agencies became more organized and had improved their law enforcement abilities, but the private investigation field continued to grow. Many people worked as private detectives and guards, and they all carried weapons. As the profession grew, the government recognized the need to regulate what private investigators can and cannot do.
Police departments sometimes hire private investigators to assist with cases because investigators are not limited by the same rules and regulations as police. Private investigators cannot make arrests, however, and cannot impersonate police; after they have turned over the information and evidence they have gathered, it it the police who are authorized to make the arrests. Many states now require private investigators to be licensed and licensing requirements vary by state. Private investigators must also adhere to current privacy laws, which prevent such things as taping or recording a person's conversation without their consent, trespassing on property, and tampering with mail.
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