WASHINGTON — Eight days before Hillary Clinton took office as secretary of state in January 2009, an aide to former President Bill Clinton quietly registered a new Internet address for the couple. That trivial but deliberate online purchase is the earliest known hint of the private e-mail system that now plagues the presumptive Democratic nominee’s presidential campaign.

Buried in a footnote in a government watchdog’s report released Wednesday, the reference to the registration of clintonemail.com was an early step toward building what became the private homebrew e-mail system that has attracted an FBI investigation and raised questions about Clinton’s judgment while serving as the nation’s top diplomat.

The State Department inspector general’s release of the 83-page report provides new insights into the server: Who knew about it, its vulnerabilities and the bureaucratic mismanagement that allowed the secret system to operate outside normal channels throughout Clinton’s tenure.

The findings — more than a year in the making — also show how the use of private e-mails by Clinton and other top aides caused internal headaches for the few State Department officials who knew of its existence and for an agency that has long struggled to comply with federal cybersecurity and record-keeping requirements.

It would take six years after that simple domain registration in 2009 for Clinton to publicly acknowledge the existence of her private homebrew server, which The Associated Press first traced back to her home in Chappaqua, New York, in March 2015. Much of what is known about the system and why she used it remains clouded by the lack of documentary evidence and Clinton’s own reluctance to discuss the sensitive topic.

Over time, through media accounts and now details in the inspector general’s report, a clearer picture has emerged of Clinton’s e-mail system and its use: A basement computer, running Microsoft server software, directly connected to the Internet to handle communications between Clinton and her aides. But it is still not clear how well her system was secured at the time, especially in light of new hacking attempts disclosed by the inspector general’s report.

In the first months of Clinton’s tenure, only her most trusted political-appointee aides used or were clued into the existence of her server, according to the report.

Outside that privileged circle, other senior officials scattered across the department had “some awareness” of her use of private e-mails to communicate internally — often because her e-mails to them originated from a rotating cluster of private clintonemail.com addresses. Some State Department officials learned as early as March 2009 that Clinton was using a private server in the basement of her family’s home.

Clinton declined to be interviewed for the inspector general report — despite Clinton saying as recently as this month that she was happy to “talk to anybody, anytime” about the matter and would encourage her staff to do the same. Three former senior aides, Huma Abedin, Cheryl Mills and Jake Sullivan, also declined.

A fourth former top aide, Thomas Nides, did not reply to the inspector general’s requests. Abedin and Sullivan are now Clinton campaign aides and Nides, currently vice chairman of the Morgan Stanley financial services firm, is a major Clinton fundraiser.