Secretary of State Hillary Rodham Clinton makes a statement on the WikiLeaks document release on Monday in Washington.

Washington ? Striking back, the Obama administration branded the WikiLeaks release of more than a quarter-million sensitive files an attack on the United States Monday and raised the prospect of criminal prosecutions in connection with the exposure. The Pentagon detailed new security safeguards, including restraints on small computer flash drives, to make it harder for any one person to copy and reveal so many secrets.

The young Army Pfc. suspected of stealing the diplomatic memos, many of them classified, and feeding them to WikiLeaks may have defeated Pentagon security systems using little more than a Lady Gaga CD and a portable computer memory stick.

The soldier, Bradley Manning, has not been charged in the latest release of internal U.S. government documents. But officials said he is the prime suspect partly because of his own description of how he pulled off a staggering heist of classified and restricted material.

“No one suspected a thing,” Manning told a confidant afterward, according to a log of his computer chat published by Wired.com. “I didn’t even have to hide anything.”

Secretary of State Hillary Rodham Clinton asserted Monday that WikiLeaks acted illegally in posting the material. She said the administration was taking “aggressive steps to hold responsible those who stole this information.”

Attorney General Eric Holder said the government was mounting a criminal investigation, and the Pentagon was tightening access to information, including restricting the use of computer storage devices such as CDs and flash drives.

“This is not saber-rattling,” Holder said. Anyone found to have broken American law “will be held responsible.”

Holder said the latest disclosure, involving classified and sensitive State Department documents, jeopardized the security of the nation, its diplomats, intelligence assets and relationships with foreign governments.

A weary-looking Clinton agreed.

“I want you to know that we are taking aggressive steps to hold responsible those who stole this information,” Clinton said. She spoke in between calls to foreign capitals to make amends for scathing and gossipy memos never meant for foreign eyes.

Manning is charged in military court with taking other classified material later published by the online clearinghouse WikiLeaks. It is not clear whether others such as WikiLeaks executives might be charged separately in civilian courts.

Clinton said the State Department was adding security protections to prevent another breach. The Pentagon, embarrassed by the apparent ease with which secret documents were passed to WikiLeaks, had detailed some of its new precautions Sunday.

Col. Dave Lapan, a Pentagon spokesman, said it was possible that many people could be held accountable if they were found to have ignored security protocols or somehow enabled the download without authorization.

A senior Defense Department official, speaking on condition of anonymity because the criminal case against Manning is pending, said he was unaware of any firings or other discipline over the security conditions at Manning’s post in Iraq.