It's become a common practice for police look into the online activities of people they are investigating, but when authorities come calling to the people who have that info, what exactly do they find out?

The Boston Phoenix has a lengthy cover story on the murder investigation of "Craigslist Killer' Philip Markoff, which is worth a read for its own sake, but also includes some interesting side information about what happens when police ask a social network to give up what it has on you. In order to catch Markoff — who was accused of murdering Julissa Brisman, and robbing two other women after hiring them for erotic services on Craigslist — police gathered a massive amount of digital evidence, tracking his online movements via email, message board postings, and his social media accounts.

As part of their story, The Phoenix obtained numerous police files via a Freedom of Information Act request and discovered that the caselog included a subpoena issued to Facebook for Markoff's profile information and Facebook's response. In addition to the technical information like login and IP data, Facebook provided Boston police with text printouts of Markoff's wall posts (both his wall and any conversations he had on other users' walls), as well as paper copies of the photos he posted and photos posted by other people that he was tagged in. It was basically a full accounting of all of Markoff's activity on the site.

Perhaps most worrying for people who aren't currently under a police investigation is that it also included a full list of the user IDs and full names of everyone the target was friends with. That means if police subpeona the profiled a criminal and you're "friends" with them, you're now permanently connected to them. Does this mean you could consorting online with a known felon?

It doesn't seem as if this particular investigation expanded to anyone beyond the main suspect, but it wouldn't take much imagination for police to use that list to start asking Facebook for more profiles, if only to widen the dragnet. If you posted a photo of criminal, maybe they want to know what else might you have posted and who else might you be talking to online?

Of course, it shouldn't be a secret at this point that nothing you put online is ever truly locked away behind a wall of privacy settings. Authorities can and will get to it if they want to. Heck, they may already know about you than you think they do. Consider this one more reminder that what you put online ever really goes away ... and you never really know who is going to see it.

The black and white printout of Markoff's photos:

Here's what the police requested: