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Days before WikiLeaks posted the classified documents, Defense Secretary Robert M. Gates sent out a memo threatening personnel with criminal prosecution if they leaked secrets.

Now the Pentagon is turning its attention to WikiLeaks, whose founder, Julian Assange, is seeking to expose all sorts of government secrets. His website also has released e-mails by climate specialists that suggested collusion and misinformation in the formation of climate change policies.

At a news conference Thursday, Pentagon press secretary Geoff Morrell said the Defense Department is asking WikiLeaks to “do the right thing” and return the classified documents it posted. However, Mr. Morrell did not say how the Pentagon would make the website do that.

Meanwhile, the Associated Press reported that WikiLeaks had posted a large encrypted file named “Insurance” to its website, sparking speculation that it was preparing another huge dump of classified documents.

Bloggers noted that the file, which consumes 1.4 gigabytes, is 20 times larger than the batch of 77,000 secret messages about Afghanistan that the website released last month.

WikiLeaks and Mr. Assange had no comment about the file, the AP reported.