Detectives have been fingerprinting suspects — and tracking criminals — for about a century. No two fingerprints are alike.

Even the same finger that’s been cut, burned, bitten, or even shorn off using sandpaper is different than it was before.

The Federal Bureau of Investigation is cautioning authorities to be on the lookout for suspects who may be trying to dodge their past by altering the crucial identifiers at their fingertips, as they said in a release last week.

Criminals who want to avoid detection will try almost any method to alter their fingerprints – even sometime enlisting medical professionals to try and change their unique markings.

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In a study last year, the FBI identified 412 records in the Integrated Automated Fingerprint Identification System which showed deliberate print alteration, they said. The biggest number of altered prints appeared to be by people who had extensive criminal records and multiple law enforcement encounters, the study found. Some were violent criminals and thieves, federal authorities said. But especially common were people who were involved drug-related offenses, and who had immigration offenses.

“Many were deported criminals who altered their fingerprints in an attempt to reenter the United States,” said the FBI, in a release.

Most of the alterations found were made by just a single vertical cut down the center of the finger, which interrupted the pattern and kept the fingerprint from having an automatic “hit” in the system. The less common kinds were a Z-shaped cut across the pad if the fingertip, burns using heat or chemicals to obliterate the print, or a combination of methods including biting, or sandpaper.

In 2012, a Michigan State University team of researchers found that altered fingerprints could regularly beat the system. They proposed developing countermeasures by cataloguing alterations, and devising a tracking method which would account for the physical changes.

The FBI asks local agencies to inspect a suspect’s hands before taking fingerprints. If they appear to be unusual, or the print submitted returns an error, they are encouraged to contact the FBI’s Criminal Justice Information Services for further tips.

Partly because of these difficulties, the feds have begun using other biomarkers — from tattoos, to eyes, to palm prints – in the Next Generation, or NGI, system, which began in September 2014.

Approximately 60 million prints were scanned looking for matches in each of the years 2012 and 2013, according to the FBI.