New York Times quotes senior administration official saying: 'They still don't know all of what he took'

Government officials have concluded that they may never know the full extent of information leaked by the National Security Agency whistleblower Edward Snowden, according to a report published on Saturday by the New York Times.

Senior government officials told the newspaper that investigators are unsure of the scope of information Snowden collected, partially because the Hawaii data facility he worked at, as a contractor, did not have employee monitoring software with which other NSA facilities were equipped. Such software is meant to detect unusual behavior among the agency's approximately 35,000 employees.

"They've spent hundreds and hundreds of man hours trying to reconstruct everything he has gotten, and they still don't know all of what he took," the Times quoted a senior administration official as saying. "I know that seems crazy, but everything with this is crazy."

Officials provided some details on how Snowden was able to avoid detection, including that he was able to hack firewalls intended to prevent employees from accessing some parts of the agency's system. They also said they believe Snowden acted alone. According to the officials, Snowden would have known the Hawaii facility did not have the employee-monitoring software installed.

On Friday, an advisory committee tasked with assessing the agency's operation submitted a report to the president. According to the White House, the contents of the report will not be made public until next month. President Barack Obama will then announce which recommendations he will act on.

Snowden, who was granted a year-long amnesty by Russia, has said that he gave all the documents, of which he kept no copies, to a group of journalists who then shared them with news organizations including the Guardian. However, the leader of the presidential advisory committee, Rick Ledgett, believes Snowden has access to documents that have not yet been disclosed. Ledgett said he would consider granting Snowden amnesty if he could provide those documents.

"So, my personal view is, yes, it's worth having a conversation about," Ledgett told CBS in an interview scheduled to air on Sunday, on 60 Minutes. "I would need assurances that the remainder of the data could be secured, and my bar for those assurances would be very high. It would be more than just an assertion on his part."