This is almost certainly bullshit. When the U.S. Government seizes something, it's public knowledge. There are court documents. There's usually a press release. There are public lists of seized items. The FBI can't put a "gag order" on a French citizen in Japan anyway.

More likely, it's more lies from Mark Karpeles. He's repeatedly blamed others for his own failure. Over the last two years, he's blamed "hackers" doing a denial of service attack against Mt. Gox. He's blamed banks for not being able to handle the "large number" of transactions Mt. Gox generated. He promised a "two week delay" on US dollar withdrawals which never ended, over half a year later. He's blamed a technical problem that didn't affect other exchanges for more than three days for a permanent shutdown on Bitcoin withdrawals. Most readers on here can add to this list.