Copperfield's lawyer and the FBI confirmed there was an investigation in Las Vegas and that investigators had been in contact with the magician, but declined to release further details.
Copperfield uses the warehouse - which he has referred to in the past as the International Museum & Library of the Conjuring Arts - to store tricks and memorabilia from around the world. Local station KLAS-TV reported that 12 FBI agents raided it Wednesday. It cited an anonymous source close to the raid.
"I can say that there was investigative activity yesterday and yesterday evening in Las Vegas," Seattle FBI Special Agent Robbie Burroughs told The Associated Press. He said the investigation was related to a case in Seattle.
Copperfield's Las Vegas attorney, David Chesnoff, confirmed only that his client had been contacted by investigators.
"We understand there is an investigation, are in touch with the investigators, and are respecting the confidentiality of the investigation," he said.
Copperfield has been a longtime performer at the Hollywood Theater at the MGM Grand casino-hotel on the Las Vegas Strip, where he has performed in six-to-eight week stints several times a year, said MGM Mirage Inc. spokeswoman Yvette Monet.
On Wednesday night, Copperfield performed two regularly scheduled shows, which were at the end of his most recent run, she said, adding he was next scheduled to perform in Jakarta, Indonesia.