A CIA statement said the hacking of the Brennan family account was a crime carried out with malicious intent.
A high-school student claimed he was responsible and told the New York Post that he had found work-related files.
The newspaper said he was angry about US foreign policy.
His Twitter account, where he is described as 13 years old, has published redacted images of what appears to be government information.
Wikileaks announced in a tweet on Wednesday that it would release some of the information later in the day.
The documents - made available a few hours later - also included a draft on national security challenges said to have been created in 2007.
The last unfinished paragraph is headed "Damaging Leaks of Classified Information".
A 2008 letter about interrogation methods is also included, purporting to be from the vice-chairman of the Senate Select Committee on Intelligence to his fellow board members.
Wikileaks said it would release more documents "over the coming days".
The CIA said there was no indication that any of the documents released so far was classified.
"In fact, they appear to be documents that a private citizen with national security interests and expertise would be expected to possess," the statement said.
High-profile leaks including a wealth of diplomatic cables from Wikileaks have dogged the US government in recent years.
The use of private email has become a major issue in recent months for White House hopeful Hilary Clinton, since it emerged that she had done so while secretary of state.
Mr Brennan became director of the CIA in 2013, having spent four years as assistant to the president for homeland security and counterterrorism.
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