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Many e-mail providers, such as Yahoo and Google, store data on their servers for a period of time, meaning that police might be able to subpoena Lanza's provider for access to whatever data they have. Google also stores information about users' searches and other online activity indefinitely, although it anonymizes IP addresses after 9 months, making it impossible to tell what a given user was doing online prior to that time.
Google says it tries to strike a balance between respecting users' online privacy and complying with government requests for information, and this might be a case where the company deems it important to cooperate with law enforcement.
Early reports of the shooting named Lanza's older brother Ryan as a possible suspect, and several news outlets mistakenly published pictures of Ryan taken from his Facebook account.
In the absence of the kind of public journaling often seen on social media sites, law enforcement will have to rely more heavily on interviews and other sources to determine a possible motive for the shooting.
Lanza seems to have guarded his digital privacy pretty closely -- but as we've seen in previous cases involving computer privacy, it's awfully difficult not to leave behind some kind of trail of e-mails and online behavior. Even if Lanza's hard drive yields no clues, investigators could still find a way to reconstruct some of his activity in the days and months leading up to the shooting.