(NEW YORK) — As the investigation into Sunday’s apparent assassination attempt on former President Donald Trump continues, the FBI is turning its attention to the obliterated serial number of the recovered assault-style weapon, using highly specialized techniques to uncover what’s been altered on the surface of the rifle.

Investigators are working to understand how suspected gunman Ryan Wesley Routh, who, according to court records, has a felony criminal history, allegedly obtained the semi-automatic SKS rifle.

In the tree-lined, chain link fenced area surrounding the Trump International Golf Club where the suspect was spotted by Secret Service personnel Sunday, agents found a digital camera, two bags, including a backpack and a loaded SKS-style 7.62×39 caliber rifle with a scope, according to a criminal complaint released Monday.

The serial number on the rifle “was obliterated and unreadable to the naked eye,” the complaint states.

Analysis into firearms includes conducting an urgent firearms trace. But to begin a gun trace, investigators need a serial number, and in this case, that key information was allegedly obliterated.

However, agents have several forensic techniques they can employ to restore obliterated serial numbers from a firearm.

Sources told ABC News Monday the FBI is forensically examining the firearm at its lab.

How investigators could recover an ‘obliterated’ serial number

Firearms manufactured in or imported to the U.S. are required by law to have a conspicuously engraved, cast or stamped serial number.

SKS-type firearms are not manufactured in the U.S.; they are typically manufactured in Russia or China and imported to the U.S. with stamped serial numbers, according to firearms expert and retired ATF executive Scott Sweetow.

“When the metal is stamped, and the deeper the stamp was originally, the more likely that the metal is to be deformed a significant amount below the surface,” Sweetow said in an interview with ABC News.

“And even if you take a grinder or scratch it out, or try to sand it out, those markings, the impression and the metal were deformed from the original serial number stamping process … those markings are going to typically survive,” Sweetow added.

Using a combination of specialized chemicals and instruments, investigators can reveal the serial numbers that, to the human eye, appear to be permanently removed, according to Sweetow.

As part of the process, chemical treatments are applied to “eat away” some of the defacement, grinding or scratching that was done to obscure the serial number, which is followed by using special instrumentation to view the previously invisible numbers “for what I would almost describe as a shadow that’s left in the metal where it was pressed down,” Sweetow said.

FBI and ATF also possess more advanced capabilities, including x-ray and magnetic resonance imaging, to peer deeper into the metal beyond what can be seen at the surface.

Though obliterated serial numbers can pose a challenge, investigators frequently overcome criminal efforts to hide the numbers.

“It certainly makes it a little tougher for investigators, but so many people obliterate serial numbers now or deface face them that the forensic techniques have gotten actually pretty good, to restore them much better than they were, say, 20 years ago,” Sweetow said.

Routh’s alleged possession of a firearm by a convicted felon carries a potential sentence of 15 years in prison, and the possession of a firearm with an obliterated serial number can carry an additional five years if convicted, according to federal statute.

Routh appeared in West Palm Beach federal court on Monday morning. Prosecutors said he is charged with possession of a firearm as a convicted felon and possession of a firearm with an obliterated serial number.

Routh did not enter a plea to the charges and was ordered to return to court on Sept. 23 for a pre-detention hearing. His arraignment has been scheduled for Sept. 30.

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