"Why didn't they do something about it?" he tweeted, adding that Mr Obama had been told about it before the vote. A hacker using the moniker Guccifer 2.0 - later linked by security experts to Russian Federation - claimed credit for the leaks, but others did too, leaving the culprits unclear.
The head of the Department of Homeland Security warned state officials on Saturday that the threat from Russian Federation targeting U.S. elections has not dissipated.
"This indictment could have been brought by ordinary prosecutors, ordinary Federal Bureau of Investigation agents, there's no conflict here, it's Russians they are going after", he added.
However the hacking allegations have sparked a heated war of words between Washington and Moscow.
Russia's foreign ministry described the allegations as a "heap of conspiracy schemes" meant to "damage the atmosphere" before Monday's talks.
The Kremlin said there was no evidence linking the 12 to military intelligence or hacking.
The 29-page indictment accuses members of the Russian military intelligence agency known as the GRU of carrying out "large-scale cyber operations" to steal Clinton campaign and Democratic Party documents and emails.
By tracing control of email and social media accounts and a tool for remote internet connections, the 29-page indictment document for the first time showed that the same group of Russians leased servers, targeted Democratic officials with phishing tricks aimed at capturing their online credentials and communicated with Republicans and other distributors of hacked information.
Deputy Attorney General Rod Rosenstein said Friday that he informed Trump of the indictments earlier in the week.
In addition to targeting the DNC, the Department of Justice alleges the Russian intelligence operatives also targeted: "state boards of elections, secretaries of state, and USA companies that supplied software and other technology related to the administration of elections to steal voter data stored on those computers".
Precisely what US intelligence agencies and former President Barack Obama knew of the Russian officials' alleged actions at the time is unclear.
"Secondly, if the president and his team are not willing to make the facts of this indictment at top priority of the meeting in Helsinki, then the summit should be cancelled", the Democratic senator said. "He is not a friend of the United States".
Friday's announcement came just a few days before Trump's scheduled meeting with Russian President Vladimir Putin in Helsinki, Finland.
"One unit engaged by stealing information, a different unit by disseminating the information", Rosenstein said.
Rod Rosenstein, the deputy attorney general in charge of the Trump/Russia investigation, called a surprise press conference for 11:45 a.m. ET on July 13, 2018 and announced the indictments of 12 Russian hackers.
Rosenstein, who is now in charge of overseeing the Mueller probe, after Attorney General Jeff Sessions recused himself in March of 2017, has been facing backlash from the GOP in the House for not readily releasing a trove of documents, some related to the Mueller probe itself.