WASHINGTON — Striking back, the Obama administration branded the WikiLeaks release of more than a quarter-million sensitive files an attack on the United States on 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 private 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.”

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.

In his Internet chat, Manning described the conditions as lax to the point that he could bring a homemade music CD to work with him, erase the music and replace it with secrets.

He told the computer hacker who would later turn him in that he lip-synched along with pop singer Lady Gaga’s hit “Telephone” while making off with “possibly the largest data spillage in American history.”

Wired.com published a partial log of Manning’s discussions with hacker R. Adrian Lamo in June. “Weak servers, weak logging, weak physical security, weak counterintelligence, inattentive signal analysis,” Manning wrote. “A perfect storm.”

His motive, according to the chat logs: “I want people to see the truth … because without information, you cannot make informed decisions as a public.”

his own admission, Manning was apparently able to pull material from outside the Pentagon, including documents he had little obvious reason to see.

He was arrested shortly after those chats last spring. He was moved in July to the Quantico Marine Corps Base in Virginia to await trial on the earlier charges and could face up to 52 years in a military prison if convicted.

There are no new charges, and none are likely at least until after a panel evaluates Manning’s mental fitness early next year, said Lt. Col. Rob Manning, spokesman for the Military District of Washington. He is no relation to Bradley Manning.

Manning’s civilian lawyer, David E. Combs, declined comment.

Federal authorities, meanwhile, are investigating whether WikiLeaks founder Julian Assange can be charged under the Espionage Act, sources familiar with the inquiry said Monday.

Attorney General Eric Holder said the Justice Department and Pentagon are conducting “an active, ongoing criminal investigation.”

Former prosecutors cautioned that prosecutions involving leaked classified information are difficult because the Espionage Act is a 1917 statute that preceded Supreme Court cases that expanded First Amendment protections. The government also would have to persuade another country to turn over Assange, who is outside the United States.

But the sources, who spoke on the condition of anonymity because the inquiry is rapidly unfolding, said charges could be filed under the act.