Magnifier May Crack Crimes, Crashes

Presidencies have teetered because of information captured on audiotapes. Tape-recorded evidence of Richard Nixon’s Watergate misdeeds led to his 1974 resignation. More recently, information gathered secretly on tapes in the Monica Lewinsky scandal almost unseated President Clinton.

Tampering clue? (Left) Magnetoresistive image of FBI sample shows magnetic ridge (arrow) created as audiotape passes beneath erase head. (Right) Although tape later passes under the write head, ridge (arrow) remains. Pappas

Although erased and overwritten with an agency logo, original hard disk data remain faintly visible in magnetoresistive scan. Pappas

A device under development by government scientists in Colorado may soon help investigators both judge the authenticity of magnetically recorded evidence and glean information from magnetic media that have been damaged—intentionally or otherwise.

David P. Pappas of the National Institute of Standards and Technology (NIST) in Boulder and his colleagues have come up with a microscope that can reveal the landscape of magnetic bumps and dips found on recorded audiotapes, floppy disks, and other sorts of magnetic media. The instrument, a type of so-called magnetoresistive microscope, slowly and repeatedly passes a sample piece of tape or other medium under a tiny read-write head from a computer hard drive. As the sample moves back and forth, the head detects the strength and direction of the magnetic field at millions of points. A computer then can make a topographic image from the data or interpret the data directly—into sound, for instance.

Physicists at the University of California, San Diego invented the first scanning magnetoresistive microscope in 1996, primarily as a tool for the hard disk industry. The Colorado researchers have adapted the device for forensic use in collaboration with the FBI and the National Transportation Safety Board.

Investigators of an airplane crash or explosion may want to play back “little scraps of tape,” such as cockpit-recorder tape, says the microscope’s codeveloper Stephen D. Voran of the Institute for Telecommunication Sciences, also in Boulder.

In one proof-of-principle test, the researchers recovered a second of music from a fragment of abused tape. The team has also read digital data encoded in a tape snippet from a flight-data recorder, or black box.

The swaths of magnetic field recorded by ordinary tape deck heads are hundreds of times wider than the microscope’s head requires. So, even if much of a tape’s surface is destroyed, the new instrument might still find a meandering path that retains the original signal, says Pappas. “Imagine a motorcycle driving around potholes,” he suggests.

An erase head leaves some inaudible marks on tape that may indicate tampering. The new device shows such telltale marks in more detail than the current technique, which uses magnetic fluid, the researchers say. However, a bigger question is whether erased data can be recovered. “Only more experiments will tell,” Pappas says.

So far, only NIST and the FBI have prototype instruments. “We’re very excited about its potential,” says FBI agent James Ryan, unit chief of the agency’s Forensic Audio/Video & Image Unit in Quantico, Va.

The FBI has eagerly sought an alternative to magnetic fluid because the fluid ruins digital tapes and disks. The new magnetoresistive microscope is the only tool that Ryan knows of that might help investigators detect certain manipulations of those media, he says.

Ryan expects to attend a meeting this fall on the infamous ’18-minute gap’—an erased section of the Watergate tapes. The keepers of the Nixon tapes at the National Archives and Records Administration in College Park, Md., are considering whether to reexamine the tape gap with new technologies. Ryan says he thinks the new microscope could be among them.

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