 -- Federal officials arrested a contractor for the National Security Agency for allegedly pilfering highly classified material that, if made public, could have caused "exceptionally grave damage to the national security of the United States."

In a criminal complaint filed in late August but unsealed today, the FBI alleges that 51-year-old Harold Martin of Maryland had kept the classified information, some of it with the highest Sensitive Compartmented Information (SCI) clearance rating, in his home or two nearby sheds. The papers and digital media files indicate they were produced in 2014, the complaint says.

"These six documents were produced through sensitive government sources, methods, and capabilities, which are critical to a wide variety of national security issues," the complaint says.

The complaint does not specifically identify which U.S. government agency for whom Martin was contracting, but a source familiar with the case said it was the NSA -- the same agency that suffered the biggest leak of information in U.S. intelligence history when contractor Edward Snowden made off with a cache of documents in 2013. The NSA declined to comment for this report.

The complaint says Martin was interviewed by federal agents in late August and, when "confronted with specific documents, admitted he took documents and digital files from his work assignment to his residence and vehicle that he knew were classified." Martin allegedly said he knew what he had done was wrong.

The White House said today that the situation was one "President Obama takes quite seriously."

The New York Times, which first reported the arrest, said Martin worked for NSA contractor Booz Allen Hamilton, the same firm that employed Snowden. Without naming Martin, Booz Allen released a statement today saying that when it learned of the arrest, the firm reached out to law enforcement to offer its cooperation and "fired the employee."

"We continue to cooperate fully with the government on its investigation into this serious matter," the company said.

The Department of Justice said that if convicted, Martin could see up to 11 years in prison for the removal of classified material and the theft of government property.

A public defender assigned to Martin told ABC News that there is "no evidence that Hal Martin has betrayed his country."

"This is the beginning of a long and important process," Wyda said. "What we do know is that Hal Martin loves his family and his country. He served our nation honorably in the United States Navy, and he has devoted his entire career to serving and protecting America. We look forward to defending Hal Martin in court.”

Military records for Martin provided by the Navy show that he served for 13 years, part of that time aboard the USS Seattle.

Former NSA general counsel Raj De said there are some key questions that need to be answered as the case moves forward, like whether Martin allegedly intended to disseminate the information or if a foreign government was somehow involved.

"We know a little bit from the criminal complaint but we don't really know the full scope of what was intended to be accomplished here," said De. "Those are going to be some key factors to thinking through how significant this event is in the grand scheme of things."

Based on initial reporting, Chris Inglis, who was deputy director of the NSA until 2014, said that while it was "not great news" for the NSA, the fact that Martin has not been charged with espionage could indicate it's not another catastrophic security breach.

"It's not a repeat of Snowden, but it is another insider," Inglis told ABC News.

Neighbors near an address listed for Martin said he was a quiet man, but described a dramatic scene when the FBI raided the home in late August. They said a loud bang went off and from the looks of it, "You would think World War III had started," as neighbor Murray Bennett put it.

"I saw all these cars and vans. The next thing I know I see FBI jackets," another neighbor, Glen Bond, told ABC News. "It was weird, especially on a Saturday afternoon... Craziness!"

Bennett said Martin and his wife have lived next door to him for more than 10 years.

"We would pass out Halloween candy together. I was looking forward to doing that together but it won't happen this year," Bennett said.

ABC News' Ali Dukakis and Margaret Chadbourn contributed to this report.