The one tail number that the initial Washington Post article linked to is N39MY[1], which is a Cessna 182T registered to NG Research, PO Box 722 in Bristow, VA. That company's web presence is close to zero, basically below the noise floor.

If you google [po box bristow va] you find FAA records for a bunch of other oddly named companies that all have similarly close-to-zero web presence and addresses that are PO Boxes in Bristow: FVX Research, NBR Aviation, NBY Productions, OBR Leasing, OTV Leasing, PSL Surveys, PXW Services. They all seem to like Cessna 182Ts.

If you Google the tail numbers of aircraft registered to those companies, you start to find forum and mailing list posts (often at sites that tilt toward paranoid/conspiracy/right wing, but not always) with people discussing these specific tail numbers and linking them to the FBI. Some of the supposed evidence includes details of radio communications that people have heard, e.g. talking about "being on station" or using callsigns that start with JENNA, JENA or ROSS, which are supposedly used by the FBI. Other posts claim that DOJ/FBI surveillance aircraft often squawk 4414 or 4415 on their transponders.

I monitor aircraft in Los Angeles using an RTL-SDR dongle. I keep a database of almost every transponder ping I receive. You can see some more info, analysis and examples of stuff I've seen (U-2, AF1, AF2, EXEC-1F, E-6 "Doomsday" planes) at http://viewer.gorilla-repl.org/view.html?source=github&user=... I decided to check my database for planes that have squawked 4414/4415 or used one of the suspicious callsigns: I found 8 aircraft in the past 2 months, several of which exhibit suspicious behavior: Flying for hours at a time without going anywhere in particular (I don't have position information for them, but I know they're in the air and not leaving the LA area), flying almost every day for months at a time, squawking 4414 or 4415, and one that used a JENNA callsign. 2 of them are registered to companies with PO Boxes in Bristow, VA. Another is registered to AEROGRAPHICS INC. 10678 AVIATION LN, MANASSAS VIRGINIA, which googling shows has also been linked to the FBI/DOJ. Several others are registered to WORLDWIDE AIRCRAFT LEASING CORP and NATIONAL AIRCRAFT LEASING CORP in Delaware, similar to other suspected FBI front companies (e.g. Northwest Aircraft Leasing Corp. in Newark, Delaware[2]).

(I call what I'm doing "persistent sousveillance": using historical sensor data to retroactively identify and track new subjects, it's just that my subjects are the government. One of the surprising things I've found is that all you need to do is look: the weird stuff jumps out right away, e.g. Cessnas registered to fake-sounding companies that loiter overhead for hours every day.)

It's a lot of circumstantial evidence, but at this point it doesn't seem far-fetched that I'm monitoring aircraft involved in persistent FBI aerial surveillance.

My twitter has more info: https://twitter.com/lemonodor/status/595814966382469120

Edit: One other thing worth mentioning is that I was surprised at how many local news stories I turned up while googling these planes & companies that fit the template of "Citizens complain about mystery Cessna flying low, circling over their neighborhood".)

[1] http://registry.faa.gov/aircraftinquiry/NNum_Results.aspx?NN...
[2] http://www.wired.com/2006/06/mystery_planes_/