By FRANK THORP V and M. ALEX JOHNSON, NBC News
(NBC News) - Two of the four classified messages discovered in emails turned over to the State Department by Democratic presidential candidate Hillary Clinton were labeled "top secret," the chairman of the Senate Judiciary Committee said Tuesday.
Clinton's emails have been under scrutiny since it was revealed that she used a private server in her home to send and receive messages when she was secretary of state.
The FBI has been looking into the security of the private server — which the Clinton campaign said Tuesday was being turned over to the Justice Department — and investigators have been trying to determine whether Clinton sent or received classified information on an unsecured system.
Sen. Chuck Grassley, R-Iowa, chairman of the Judiciary Committee, said Tuesday night that Charles McCullough, the inspector general for U.S. intelligence agencies, had reported that two of the emails not only were classified but were in fact categorized as "Top Secret, Sensitive Compartmented Information" — one of the strictest security classifications.
Grassley has been pushing for details of the classified emails found on Clinton's private server and a thumb drive in the possession of her attorney. Clinton's turning over of the server is "a welcome development," he said, but he declared: "That's a long time for top secret classified information to be held by an unauthorized person outside of an approved, secure government facility."
Clinton aides have maintained that nothing on her server was classified at the time she saw it, suggesting that classified messages were given the label after the fact.
John Kirby, a spokesman for the State Department, said that was the case with two emails, adding that it remained unclear "whether, in fact, this material is actually classified."
"Department employees circulated these emails on unclassified systems in 2009 and 2011, and ultimately some were forwarded to Secretary Clinton," Kirby said Tuesday. "They were not marked as classified."
Nick Merrill, the Clinton campaign's press secretary, said Clinton had "pledged to cooperate with the government's security inquiry, and if there are more questions, we will continue to address them."
Merrill said that since Clinton asked the State Department to publish the emails in the spring, she had also turned the server itself over to the Justice Department.
House Speaker John Boehner, R-Ohio, said "it's about time" Clinton handed over the server.
"Secretary Clinton's previous statements that she possessed no classified information were patently untrue," Boehner said. "Her mishandling of classified information must be fully investigated."
Rep. Trey Gowdy, chairman of the House committee investigating the Sept. 11, 2012, attack on the U.S. consulate in Benghazi, Libya, said the information represented a "serious national security issue."
"The revelation that Secretary Clinton exclusively used private email for official public business, and the multitude of issues that emanated from her decision, including this most recent one, demonstrates what can happen when Congress and those equally committed to exposing the truth, doggedly pursue facts and follow them," he said.