A keyword search through LegalTrac on the phrase "private investigator" returned 27 citations, from 1984 to the present, from prominent publications, including California Lawyer, National Law Journal and Bench and Bar. Here's one abstract.
The National Law Journal, March 10, 1997 v19 n28 pA1 col 2 (63 col in)
Use of private investigators by lawyers on the rise; but sleuths say many attorneys don't know how to use them effectively. Loren Stein.
Abstract: The success of both civil and criminal litigation depends more on a partnership of lawyer and private investigator as litigation grows in complexity. Demand for investigators to perform services in addition to the ones they used to be responsible for has increased and according to one survey their use has increased 40% in the last 5 years. Advantages of using investigators include their decreasing the pretrial discovery process and their ability to testify as neutral witnesses to a jury.