An aide who played a central role in the set-up of Hillary Clinton's private email server said Tuesday during a House Oversight Committee hearing that he had constant access to the network despite holding no security clearances.

Justin Cooper, a former Clinton Foundation advisor, faced questions about why he destroyed two of Clinton's Blackberry phones with a hammer.

"I was going out of my way to preserve all of the information that was on those devices," Cooper said, denying that he smashed the phones in an effort to conceal records. "We would take the information that was on the old device, back it up, transfer it to the new device."

The revelation that Cooper took a hammer to Clinton's Blackberrys came in 58 pages of notes made public on Sept. 2 from the FBI's investigation of Clinton.

Those notes revealed Bryan Pagliano, the IT aide who provided maintenance to the network during Clinton's State Department tenure, knew classified information would likely pass through the server he built.

The FBI noted they had recovered more than 2,000 classified emails from the server, including three that bore classified markings. Eight email chains contained top secret intelligence at the time they were written, and hundreds of others were classified retroactively.

Cooper said he set up the "clintonemail.com" domain when "Secretary Clinton was transitioning from [her] presidential campaign and her Senate role" to the State Department. Clinton wanted a new email account because too many people knew her address from her time in the Senate and on the 2008 campaign trail, Cooper noted.

Clinton has previously defended her private email use by claiming she simply added an account to a network already in use by her husband.

Cooper told the committee he was interviewed three separate times by the FBI in the course of its year-long investigation of the server system.

The former foundation official was the only witness with knowledge of the email network to testify at the oversight hearing Tuesday.

Three others were invited, but Pagliano ignored a subpoena to appear and two others exercised their Fifth Amendment rights and refused to answer questions.

Although Cooper was listed as the registrant of the "clintonemail.com" domain, the extent of his involvement in the email controversy was unclear until several of Clinton's aides gave depositions to conservative-leaning Judicial Watch in an open records lawsuit. Several of the aides, including Huma Abedin and Cheryl Mills, pointed to Cooper as one of the managers of the server.

The House Oversight Committee has been agressive in its pursuit of records related to the FBI's controversial handling of the Clinton investigation, stepping up its inquiries after the FBI notes revealed a pattern of evasive behavior from the Democratic nominee and her staff.

FBI Director James Comey concluded his agents did not find sufficient evidence of criminal intent during their probe, an outcome that received renewed scrutiny in light of the summary report.