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Relying on computer evidence seized from his Manhattan home last October, the district attorneys in Manhattan, Brooklyn and Queens have charged Mr. Ket under his real name, Alain Maridueña, with 14 criminal counts, including trespass, criminal mischief and making graffiti. If convicted, he could potentially face decades in prison and huge financial penalties.

Mr. Maridueña is charged with painting several recent images on subway cars, a form of graffiti vandalism that has largely died out since the early 1990s. But his case, and his recent professional history, underscore how graffiti has been propelled from the shadowy corners of the subway system into a global genre of virtual images circulated on the Internet, and become a powerful influence in design, fashion and graphics.

The case could pose an important test for prosecutors and the police, since Mr. Maridueña was never caught in the act and has no previous criminal record in New York City. Instead, the government’s case appears to be based largely on what prosecutors say is the unmistakable detail of his graffiti signature — his “KET” tag — and the fact that the tag is visible on photographs of illegal subway graffiti that were entered into Mr. Maridueña’s home computer only hours after identical work was discovered on subway cars.

Read More: Graffiti Figure Admired as Artist Now Faces Vandalism Charges