At a glance, the Gibson guitars look like some of the most coveted six-stringed instruments in the world, including signature models played by Dave Grohl that sell for up to $10,000 and some that looked as if they had been autographed by Slash.

A closer look by any guitar connoisseur, though, would reveal something was slightly off.

Federal officials aren’t saying how they knew the difference, but the 3,000 electric guitars that arrived in ocean containers from Asia at the Los Angeles/Long Beach Seaport were counterfeit.

Their total retail price, had they been genuine, would have been $18 million, they said. The haul is the largest seizure of counterfeit musical instruments on record, officials said in announcing the bust on Tuesday.

“These fraudulent guitars may look and feel legitimate for unsuspecting consumers buying them from third-party online sources, street markets, unauthorized retailers, and person-to-person transactions,” said Cheryl M. Davies, the director of field operations for Customs and Border Protection in Los Angeles.

The biggest tip-off, though, might have been that authentic Gibson guitars are made only in Nashville and Bozeman, Mont.

“This is really emotional and personal for us,” Beth Heidt, chief marketing officer at Gibson, said at a news conference in Los Angeles on Tuesday. She said that generations of American families “have dedicated their entire lives to hand-crafting Gibson instruments.”