Besides targeting voting systems for vulnerabilities, USA intelligence committees have said Russian hackers hacked emails from the Democratic National Committee and Hillary Clinton campaign chairman John Podesta.
In his testimony, Johnson said he was "anxious to know whether or not our folks were in there". "The FBI and DNC had been in contact with each other months before about the intrusion, and the DNC did not feel it needed DHS's assistance at that time". "Homeland Security does not engage in election recounts", he said, under questioning from Representative Jackie Speier, a California Democrat.
CrowdStrike, the private security firm hired by the DNC, eventually attributed the breach to Russian actors - a conclusion now supported by 17 US intelligence agencies.
Johnson served as former President Barack Obama's homeland security chief for three years.
An intelligence report released in January concluded that "Russian President Vladimir Putin ordered an influence campaign in 2016 aimed at the US presidential election" and acted "to undermine public faith in the USA democratic process, denigrate Secretary [Hillary] Clinton and harm her electability and potential presidency".
Though Johnson declined to say in a public setting how the intelligence community reached that conclusion, he said the intelligence made it "a pretty clear case".
"Perhaps beyond a reasonable doubt", he added.
Johnson, now a partner at Paul, Weiss, Rifkind, Wharton & Garrison LLP in NY, at Wednesday's hearing described his experience in uncovering Russia's efforts to undermine the 2016 USA election.
When he convened a conference call of state officials August 15 of previous year to tell them he was planning to designate the country's election system as part of the nation's "critical infrastructure" - a move that would have provided extensive federal cybersecurity protections similar to that given to power grids, dams and financial services - the reaction, "to my disappointment", was "neutral to negative", Johnson says.
"Those who expressed negative views stated that running elections in this country was the sovereign and exclusive responsibility of the states, and they did not want federal intrusion, a federal takeover, or federal regulation of that process", Johnson's testimony states.
In January DHS declared election systems to be critical infrastructure that the department must prioritize, but the hearing showed there are still questions and concerns about what that exactly entails.
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But he ultimately chose to wait until January to make the designation, calculating that it would be a counterproductive to do it before then.
Johnson counted 33 states and 36 cities and counties as having made use of Homeland Security tools to scan for potential vulnerabilities. He also details a largely failed effort to get elections system designated as critical government infrastructure - which would have given greater priority to states seeking help in possible cyberattacks.
Wasserman Schultz has not commented on Johnson's testimony.
Jeanette Manfra, Homeland Security undersecretary for cybersecurity, said there is evidence that 21 state election systems were targeted, but she told the Senate intelligence committee she couldn't disclose the identities of the states because that was up to the states.
Johnson encouraged as much himself on September 16, urging states to be vigilant in the face of suspicious cyberintrusions of political institutions and voter-registration data.
Johnson said, "I have no knowledge that votes were altered or suppressed in some way".
The Obama administration was particularly concerned about being seen as interjecting into the election during a heated campaign, or taking steps to delegitimize the election process. Johnson released a joint statement with former Director of National Intelligence James Clapper on October 7 accusing Russian Federation of being behind the attacks. But that statement he said "did not get the attention it should have", because of the release of the Access Hollywood tape that day - in which then-candidate Trump boasted of groping and kissing women.
But though the government disclosed that 21 states were potentially impacted by the targeting, lawmakers were left frustrated that the public still doesn't have a full picture of what exactly the Russians did during the election and that it's not fully clear what the USA will do to protect itself going forward.
It's been said many times, but witnesses reiterated they have no doubt that Russian Federation itself was behind efforts to intervene in the U.S. election and were also very confident that no votes were changed in the process.
He said Russia's efforts convinced him to sign onto an October 7 statement publicly blaming the Kremlin for what had happened, even though that might be perceived as "taking sides" or "challenging the integrity of the election itself".