You can back this up with actual, linked citations of some sort, of course.
You can start here
From another article:
.... One of its sources was Roy Bullock, a person who collected information and provided it to the ADL as a secretly-paid independent contractor over 32 years. Bullock often wrote letters to various groups and forwarded copies of their replies to the ADL, clipped articles from newspapers and magazines, and maintained files on his computer. He also used less orthodox, and possibly illegal, methods such as combing through trash and tapping into the White Aryan Resistance's phone message system to find evidence of hate crimes. Some of the information he obtained and then passed on to the ADL came from confidential documents (including intelligence files on various Nazi groups and driver's license records and other personal information on nearly 1,400 people) that were given to him by San Francisco police officer Tom Gerard. (Richard C. Paddock, "New Details of Extensive ADL Spy Operation Emerge," Los Angeles Times, April 13, 1993, A1)