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Police and federal investigators are still

searching for motives behind Adam Lanza's killing spree in Newtown, Conn., last week. One important piece of evidence: Lanza's computer hard drive, which investigators say the 20-year-old smashed with a hammer or screwdriver before the attack.

Presumably, Lanza wrecked his hard drive to prevent police from prying into his files. But that may not have been enough to keep his secrets. The F.B.I.'s computer-analysis response team is currently working to pry data off of the broken drive.

Yes, physically destroying a hard drive renders your device and the data on it unusable. But with enough motivation and the right equipment—and the F.B.I. has both—some of your data can be recovered. Dan Kaminsky, chief scientist of security firm DKH, says 100 percent physical data destruction is nearly impossible. The only method that comes close is overwriting the disk. (Kaminsky is not involved with the Newtown case.)

"Within a hard drive, there are several square feet of [read/write] surface, and any given file takes up less than a square millimeter," Kaminsky says. "That means even if the drive is dented or shattered into tiny pieces, the actual surface that contains the data is still there and readable."

Kaminsky compares a shattered hard drive to a piece of paper that is torn apart. If all of the pieces are still there, you can "tape it back together" and make out what was originally written on it. To read a shattered drive, its disk will need to be reassembled, which is a daunting but doable task.

"There aren't many systems capable of this level of reverse engineering," Kaminsky says. "Every manufacturer customizes its hard drive to store data differently. Information can be encoded in thousands of ways, tailored to a particular pattern that's physically compatible to the unique surface you're writing to. To extract the data, you would need to reassemble the pieces and reverse the manufacturer's transformation."

Investigators can identify the make and model from a hard drive, and then request the proprietary details from the manufacturer. "There's a tremendous amount of information about hard drives that's shared between forensics professionals and not the public," Kaminsky says.

This kind of complicated restoration is very different than what pros would do to rescue data from a crashed drive. In that case, the drive's read head has usually stopped working and the disk has stopped spinning. Recovery engineers remove the disk, which is stored on a platter, and put it on a larger, dedicated reader.

In the Newtown case, the physical integrity of the platter itself is suspect. The severity of damage is not yet known, but if investigators are lucky Lanza may have only used enough force to stop the disk from spinning, rendering it silent but only dented. Data on dented plates can be accessed much quicker than data on shattered drives.

The number of computers Lanza owned, and what he did on them besides play video games, is still unknown.

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