US Defence Secretary Robert Gates has called in the FBI to help with the invesitgation after more than 90,000 classified military records were leaked. He has asked the FBI to assist the Army's probe to ensure that the investigation "can go wherever it needs to go."
The Pentagon has said that the whistle-blowing website WikiLeaks may have blood on its hands, warning its unprecedented leak of secret US military files could cost lives and damage trust of allies.
An Army intelligence officer, already under arrest, is at the center of an investigation into the leak of more than 90,000 secret records to WikiLeaks, one of the biggest security breaches in US military history.
Defense Secretary Robert Gates declined to comment on the probe but said he could not rule out more leaks of classified information. He also announced plans to tighten access to sensitive intelligence data.
"I don't know whether there is anyone else out there that is a party to this," Gates said at the Pentagon in his first public comments since Sunday's publication of the documents.
Admiral Mike Mullen, who as chairman of the Joint Chiefs of Staff is the top US military officer, lashed out at WikiLeaks' founder Julian Assange, who says he aims to expose corporate and government corruption.
"Mr Assange can say whatever he likes about the greater good he thinks he and his source are doing," Mullen said. "But the truth is they might already have on their hands the blood of some young soldier or that of an Afghan family."
Gates said he did not know whether Assange should face criminal prosecution or whether WikiLeaks should be treated like a media organization protected by free speech rights under the US Constitution. "I think that's a question for people who are more expert in the law than I am," he said.
