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.
These poor helpless Gox customers are just desperate, but understandable. For the amount of money they can't get back there is not even a sound explanation.
Actually as a poor helpless mtgox customer (who thank god can live easily without the small amount of coin i lost) this theory scares me even more than mtgox loosing a fair chunk of the money. The USG can and does make life hell for people with little or no reason. Just ask anyone caught up on Megauploads downfall. The USG just said one day fuck everyone and the files are simply gone. They still havent bothered putting together much of a case.
If mtgox simply fucked up i have a chance of some recovery. Be it via a buyout or bankruptcy
If the USG has the cold storage my chances of ever seeing even a fraction of a bitcoin are exactly zero. They can and will drag any case on for decades and hide behind the full force of law to stop me making any recovery action.
If you think they wouldnt LOVE to fuck Bitcoin over completely your kidding yourself. Any excuse will do. Legal doesnt worry them any more especially if you throw in the war on drugs or terrorism. Two things they will surely link to Bitcoin at every opportunity.
If i had significant BTC holdings right now id worry. Especially if they are sitting on any exchange or online wallet system in a country that wont immediately tell the USG to fuck off. the second i heard that mtgox had received a subpoena from the USG in the month before it went down i suspected they were highly involved in causing its shut down.