NYT story on it:
Evidence so far suggests that the attack was the work of at least two separate agencies, each apparently working without the knowledge that the other was inside the Democrats’ computers. It is unclear how WikiLeaks obtained the email trove. But the presumption is that the intelligence agencies turned it over, either directly or through an intermediary. Moreover, the timing of the release, between the end of the Republican convention and the beginning of the Democratic one, seems too well-planned to be coincidental.
http://www.nytimes.com/2016/07/25/us/politics/donald-trump-russia-emails.html